The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: lawnmowerman on 20.01. 2011 16:28

Title: Spit back through carb and backfires
Post by: lawnmowerman on 20.01. 2011 16:28
Hi All
Just spent a few days getting the SR ready for its MOT. Over the winter I have fitted western bars and longer cables, craven rack and panniers. Also fitted new BAP taps from SRM as the rubbers in the old ones had disintegrated and were causing fuel starvation and I had to run with both taps open.
Today I took the oil tank off and flushed it out, refitted it and filled up with new oil and took it outside and fired it up. It started ok - well as ok as it usually does and I let it warm up for a while. When it was warm I noticed that I get a spit back through the carb when I blip the throttle which it did not do before ? only when cold.
I did not use the choke to start up as I have never found it necessary, and using the choke while running had no effect on the spit back.
Another thing it was doing was backfiring on one side when I retarded the ignition. Before it just used to miss and run on one when I retarded it and is something I have been trying to fix over the past year and has been doing it since I bought it.
Took it back in the workshop and took the plugs out and they were both sooted up right down the bottom of the central electrodes.
Took the leads off and the next step was to clean the mag slip ring with a clean bit of rag in case the spark is tracking round it and firing one cylinder on the inlet stroke causing the spit back and possibly also causing the backfire.
Just came in for a coffee so I thought I would consult the oracle for ideas before going any further.
My first thoughts with the carb spit back are a sticky inlet valve as the engine was rebuilt about 500 miles ago and they may still be a bit tight. Also I may have to bite the bullet and get the mag reconned but wanted to eliminate all other possibilities first.
Any thoughts?

Jim

Title: Re: Spit back through carb and backfires
Post by: bsa-bill on 20.01. 2011 16:35
Western bars and a rocket presumably with manual advance / retard, any chance Jim the A/R cable is now the wrong length
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 20.01. 2011 16:55
Hi Bill

Just been out to check and it looks ok - moves the cam ring betwwen the stops. I moved the lever lower towards the top yoke when I fitted the bars. Cleaned the slip ring while I was out there (ater earthing the mag) with a nice clean bit of white rag and it did come out with some carbon dust on it but probably no more that expected.
I fitted new plugs, caps, pickups and leads last year while trying to fix the missing / backfiring problem although it may have been due to fuel starvation with the old taps.

Jim
Title: Re: Spit back through carb and backfires
Post by: sinbad on 20.01. 2011 20:53
Hi Jim,thinking along Bills lines did you check the choke slide + cable. I think then I would be checking  why the very sooty plugs is it missing that badly Rod
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 20.01. 2011 23:12
Hi Rod
The choke is ok - the slide clears the bore of the carb. The plugs have always looked sooty ever since I have owned the bike. Perhaps the next step is to lower the carb needle a notch.
The bike has been sitting in a dry but unheated workshop since last September so perhaps it just needs a good run.
I am making a tank strap tomorrow which ties the two tank halves together underneath. It did not have one when I bought the bike so I ordered one and it was too short so the tank must be an Indian version.
When I have finished that I will call the local MOT shop and have a run out there for the test and see if a run and lowering the needle help the situation.

Jim
Title: Re: Spit back through carb and backfires
Post by: sinbad on 21.01. 2011 00:33
May be its to rich now cos of problem with old taps,  Talking of taps check carb for any bits if you haven't done already, just a thought.
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 21.01. 2011 10:54
Hi Rod
Yes a possibility it is too rich now it is getting a decent fuel flow from the tank although if I am honest, ever since I have had the bike the plugs have always looked a bit sooty - never the chocolate brown colour they should be. I will try dropping the needle a notch.
Good point you made about the carb being blocked by bits of rubber from  the old taps. I have checked the carb banjo filter but smaller bits may have got through and built up in the carb. With a dry carb while it has been laid up - these bits may have dried out and solidified. I will check and clean what I can without taking the carb off as I do not want to make too much work at this stage as I need to get the bike MOTd for back-up transport as the car is playing up and I live out in the sticks with few buses.

Jim
Title: Re: Spit back through carb and backfires
Post by: bsa-bill on 21.01. 2011 11:29
Quote
never the chocolate brown colour they should be.

I'm beginning to think Jim this is something your not going to see much unless after a good long run or a short sharp thrash, both these options are out for the now with my project due to running in and it not being taxed at present.
I think we need to renew our thinking on this as even after setting up the carb with a colortune the plugs still are Black, but it's not soot I believe, although as I have said a warmer engine is needed
Title: Re: Spit back through carb and backfires
Post by: cyclobutch on 21.01. 2011 17:23
Maybe the old taps were restricting flow enough that it kept her leaned out a little. Now you've got full flow to the carb again perhaps it's showing up problems there?
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 22.01. 2011 15:44
Picked up some Extralube from Halfords today so I will put some in tomorrow and see if it helps - maybe a sticky inlet valve causing the spit back through the carb.
I am in the same situation as you Bill - the engine was rebuilt about 500 miles ago so I am reluctant to give it a good thrash. I will try dropping the carb needle tomorrow and check the float level in case that is causing it. I read on another thread the the "stayup" floats have a ridge on the side which sometimes binds on the carb body - not sure if I have one in as I have not had the cover off before.
The bike does seem to be running really rich though - you can smell it and there is a sooty deposit in the end of the silencer tail pipes and it is not dry powder but quite oily as though it has been mixed with unburnt fuel. The bike does not smoke so I doubt it is unburnt engine oil.
It will be difficult to tell if things have improved without a road test so I will book an MOT and ride it there and see how it performs.
Next step will be to check the needle size and main jet size - also the slide cutaway.
I also have the misfire / backfire problem on manual retard but only on one side so should not affect both plugs. It must be a seperate problem, probably mag related.

Jim
Title: Re: Spit back through carb and backfires
Post by: Stu55Flash on 22.01. 2011 21:41
I had this spit back problem last year when i first started my plunger after rebuild. It was down to a few things that were not quite right. .

The carb inlet valve that controls the fuel level in the carb bowl was worn (variable float level) - sorted with a full Amal rebuild kit.
I did not have the air filter in place and therefore could not get things adjusted properly
I checked the timing - and found that the new auto advance springs had fallen off and where just about to go into the crank case!
The mixture was too lean when i got it running i blued the pipes - so lifted the needle. If i do it again i would prefer to start rich and lean back rather than blue the pipes. Although you may knacker some plugs in the process!

Hope this helps

Stu
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 23.01. 2011 10:40
I do not have an air filter on mine - just a short bellmouth with a gauze fliter. I wonder if the PO discarded the filter when he rebuilt the bike. I bought it rebuilt but not registered so it had not had a shakedown. Perhaps the jets need a different size as there is no filter although I would have thought that no filter would have meant more air hence a weaker mixture. Also I have changed to twin goldie silencers so may be less back pressure - not sure whether this will have too much effect on the mixture on  a four stroke. I know it has dramatic effects on a two stroke and can wreck engines - DAMHIK  *smile*
I fitted new pipes last year and they are going a very light gold colour - not sure if this is a red herring or just poor quality chrome. I have heard from others that some bluing is unavoidable if ridden hard.

I am going to put the Extralube in today and if it stops raining I will drag it outside and fire it up to see if cleaning the plugs and slipring had any effect.

Jim
Title: Re: Spit back through carb and backfires
Post by: nigeldtr on 23.01. 2011 10:54
Hello Jim,

Just a thought, have you got access to a compression tester. Might be worth a quick check just to make sure both cylinders are the same. Might also show up a leaky valve, not sure though.

Nigel
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 23.01. 2011 11:25
Hi Nigel

Thanks for your suggestion.
I do have a compression tester but in my experience they are normally used on cars to test cranking pressure using the starter motor. This ensures that the engine turns at the same speed when testing each cylinder. I suppose it would be possible to use it while kicking over the bike but in order to compare cylinders you would need more that a short kick so pressure can build up. Could try starting up on one cylinder and check the other side I suppose and I may see a pulse on the tester which coincides with a spitback. I think that if it is a sticky valve, it only happens occasionally so apart from a strip and check I will probably try the Extralube first.

Jim
Title: Re: Spit back through carb and backfires
Post by: nigeldtr on 23.01. 2011 14:41
Hello Jim,

I used to have one with a valve in, so that it built up to the maximum (just used to leave it in and kick over several times) - haven't used or seen one for at least 30 years!!

PS did you buy the SR from a guy in Liverpool?
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 23.01. 2011 15:09
Yes Nigel I got it from a guy in Liverpool through eBay just over a year ago - why do you ask?

Well I have just come back in from the workshop after firing up the bike again.

I drained the old fuel out of the tank and removed a load more Petseal which had dislodged, then refilled with some avgas.

Took the carb off and blew out the jets. The needle was on the third notch from the top so I dropped it one notch to the second from the top. Carb looked in pretty good nick - slide was a nice tight fit and operated well, as did the choke slide.

Added some Extralube to the oil in case a valve was sticky.

It would only start on one cylinder until I moved the lever to almost full advance. When it did run I went to full advance, warmed her up and moved the slide stop so it would run without using the twistgrip. She would only run at about 1500 rpm and the speed was not even ? all the time there was a random spitback through the carb. I tried blipping up to about 4000 revs and seemed to be fairly even but did not want to run it too fast for too long.

Shut her off and took her back in the workshop and checked the plugs. There was no soot but I did notice that the pipes were going a golden colour near the head. A mate of mine who rebuilds Manx Nortons called round earlier and I was talking to him about it and he reckons that it is quite normal for the Nortons to blue right round to the footrest!

Sounds to me that the timing may have been progressively slipping over the time I have had the bike. I think my next step is to check the timing and points gap also checking the gap is OK on both sides of the slip ring in case it has been poorly machined.

If that is OK I think I will be looking at a mag recon.


Jim
Title: Re: Spit back through carb and backfires
Post by: nigeldtr on 23.01. 2011 16:51
Jim,

You beat me to it! I have been after a Road Rocket for ages - small world. I am sure you will get plenty of good advice here. I would wait to see what the others say first before you do too much more. All sounds a bit suspect to me. I am no expert but if the points and cam ring are in good order and the timing is correct (have you checked this on both LH and RH cylinders) there must be a mechanical problem. I hope its something simple like a naff pickup or something.

Kind regards

Nigel
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 23.01. 2011 17:04
Thanks Nigel. Bet you're glad I did beat you to it after all the trouble I am having  *smile*

Jim
Title: Re: Spit back through carb and backfires
Post by: bsa-bill on 23.01. 2011 17:09
Jim I had something similar with my project and amongst other things the cam ring had moved back and was off it's peg a bit allowing the cam ring to move from desired position and altering the timing
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 23.01. 2011 17:30
Hi Bill

I remember reading about that in one of your earlier posts. I did take the cover off and check mine but the ring seems to move backwards and forwards using the lever and also seems flush with the outside of the mag when the cover is off. I did not want to pull the ring right out in case I had a problem getting it back. Does it sound ok or does it need further investigation - it sounds exactly like what could be causing the problem. Either that or a loose gear wheel on the mag drive.

Jim
Title: Re: Spit back through carb and backfires
Post by: bsa-bill on 23.01. 2011 17:39
Ah sorry Jim I forgot yours is cable
Title: Re: Spit back through carb and backfires
Post by: chaterlea25 on 23.01. 2011 23:00
HI lawnmowerman,
"Normally" the camring is a litle below the outer face when fitted,
They can and will move outwards if and when the engine is run *sad2*
It depends somewhat on the exact type of end cover, there were/are some pattern plastic/bakelite ones around that allowed the cam to move outwards!!
This affects the timing as the ring is no longer tied to the actuating thingy!
If you have a clip on strobe light (not one that is in series with the plug lead) you can see if the miss is ignition or not
by shining the light on the primary or timing case, its easy to spot the break in the light pulses

Last year I had an A10 in that would not run evenly at all, it turned out to be a soft valvespring, that along with the fact that it was totally knackered *eek*

HTH
John O R
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 24.01. 2011 09:47
Thanks John

The cam ring moves when I operate the A/R lever so on that basis could it have moved or be out of engagement? Is it ok to pull out the cam ring and inspect it with the points still in place. You mention that the cam ring should be slightly below the body - is there anything which holds it back or is it reliant on the cover, in which case it could creep forward until level with the body and the cover will stop it moving forward any more.
Hopefully the engine is still mechanically ok as it was professionally rebuilt recently but not sure about the mag as I cannot see any receipts for that from the PO. It ran OK last autumn apart from a misfire on the drive side which seemed to get worse when manually retarded when it would run one one side only. This was there ever since I bought the bike a year ago.
It is looking more and more like a mag problem. The first winter I had the bike I kept it in the conservatory over the winter, however this winter under pressure and threats from SWMBO I kept it in my new workshop whch is dry and well insulated but unheated so perhaps the mag was on its way out and the damp over the winter has finally killed it. The intermittent carb spit back when running has only started now after the winter layup. Perhaps I could try disconnecting the drive side plug and starting on one and see if the spit back goes - if it does it could point to arcing on the mag split ring and sending random sparks to the drive side causing the spit back.

I do have a clip on strobe which I bought for the car so I will give that a go. I will also check that the the
timing has not slipped using a rod down the plughole and the point at which the points split.

Jim
Title: Re: Spit back through carb and backfires
Post by: muskrat on 24.01. 2011 12:27
G'day Jim,
              while you have the strobe light on, check the timing is the same on both cylinders. A worn cam ring will do that and if bad enough will make one side out of time enough to misfire or spit back.
Cheers
Title: Re: Spit back through carb and backfires
Post by: nigeldtr on 24.01. 2011 18:28
Hello Jim,

Have you checked the timing on both cylinders, is it the same? If you want to take the cam ring out, just turn the engine over slowly (plug leads off) and make sure the points are closed, off the cam. At this point you can simply slide the ring in and out - just need to locate the A/R plunger in the grove when you put it back in.

Regards

Nigel
Title: Re: Spit back through carb and backfires
Post by: chaterlea25 on 24.01. 2011 23:12
HI Jim,
The cam can move out but still be engaged with the plunger (couldnt think of the name of it last nite *conf*)
As it moves towards the cover it may slip past the wee peg that limits the advance/retard range, and so the timing may be out in this position *eek*
Push the camring fully home in mid position, this will ensure it goes in correctly!!
The end cover is all that holds it in place
Sometimes the limiting peg can be damaged or bent!! if this happens symptoms like you are having can occur
ie, losing sparks on one side when the lever is towards the retarded position

(Its coming back to me now ???? ????)

Some time ago I had problems with a K2F similar to yours,
The peg which is factory set and a rivit fitted at the end so as you cant fiddle with it
in the retarded position the points were opening when the brass segment of the slipring had passed the pickup brush
I carefully drilled off the rivit head and was then able to turn the screw with the peg which is eccentric to the screw
to advance the camring.
I set it the same as another K2F I have and glued it in place with some araldite
This mag had a steel points plate, and when compared to a brass plate setup I found that the points opened sooner
in relation to armature position.
To "fix" this I filed away the leading edge of the fibre block to delay the points opening so as to make it the same as the brass points plate
Then the mag has to be retimed to the bike!!

HTH
John O R



Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 25.01. 2011 10:32
Thanks Guys - some really useful information.
I do remember soon after I got the bike that the A/R lever tightened up but I thought it may have been a corroded cable so I gave it a bit more force and it seemed to free up - perhaps I have pulled the cam ring off the stop or bent the stop.
I will have a quick look and do a rough check on the timing but I do not have a timing disc yet so will use the "straw down the plug hole" method posted by alanp a while back.
If things look ok I will pull out the cam ring and reseat it then check the points are the same gap on each side.
If I then need to take the timing cover off and rermove the mag for a rebuild I will wait until I have bought a bike lift so that I can do the job in a bit more comfort. I will also invest in a timing disc so I can strobe it when I replace the mag rather than using the static method.
I also like the idea of slotting the mag mounting holes to give some adjustment once the mag gear wheel is back on and the timing case fitted.
Thanks again

Jim
Title: Re: Spit back through carb and backfires
Post by: MG on 25.01. 2011 10:37
Hi Jim!

That sounds very much like a problemm I had with my A7. The advance lever was stuck, but then freed with some gentle persuasion. A few miles afterwards the bike would start to backfire and finally stall.
Turned out to be the (pattern) magneto end cover, that wouldn't hold the camring in place properly, so that it would move off its' stop and turn 'round in the housing. I then fitted an original Lucas cover taken off another mag and had no more trouble ever since.

Cheers, Markus

*edit*
I just read chaterlea's post above, so has obviously been mentioned before. One more good reason to check ;-)
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 25.01. 2011 11:53
Just spent half an hour in the workshop before I have to get on with non-A10 work.
I removed the mag end cap, moved the points off the cam and pulled out the cam ring. Everything looked ok - the A/R lever moved the actuator back and forth and the slots in the cam ring did not look unduly worn so I put it back. The cam ring sits just below the face of the mag and is a snug fit - checked the lever movement and the cam ring went from stop to stop.
Checked the points gap and it was the same on both lobes, albeit a little wide at 15 thou - I think it should be around 12. I could also feel a slight step on the face of the contacts so may need a little work with a points file before I reset them.
That's as far as I have got at the moment - next step is to check the static timing. I am not too bothered if it is spot on but it should show if it is way out and the drive gear wheel has slipped on its taper shaft.

Jim
Title: Re: Spit back through carb and backfires
Post by: nigeldtr on 01.02. 2011 22:13
Hello Jim,

How are you getting on - have you managed tracked down the problem yet?

Regards

Nigel
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 02.02. 2011 01:44
Hi Nigel

I have ordered a bike lift so I can work in a bit more comfort - should arrive in the next few days and then the plan is to check the timing. If it is way out then that may be the problem, otherwise it's off with the mag and get it reconned. I think it may well be a mag problem and this winter has finally killed it - I was going to remove it and put it in the oven on a low heat overnight to see if it improves things but I will probably bite the bullet and go for a recon - at least it will eliminate one area.

I will let you know what I find when I check the timing.

Jim
Title: Re: Spit back through carb and backfires
Post by: wilko on 02.02. 2011 19:06
For all it's worth, it simply sounds too retarded. Just advance your timing some more as an experiment before you lash out with more dosh!
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 02.02. 2011 22:18
Hi Wilko

It was ok before the winter layup - just the ongoing misfire on one side problem but no carb spitback or backfires so I doubt if it is ignition timing but anything is possible. When I get it on the lift later this week my plan is to check the static timing as a last resort as the gear wheel may have slipped.

Jim
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 03.02. 2011 16:09
Now I'm confused.
The bike lift still has not arrived but I could not resist going into the workshop for half an hour to do a rough check on the timing. I bent a piece of flat tin and bent it over the plug hole and secured it with zip ties to give me a datum and put a piece of thick wire through a hole and down the plug hole. I then used top gear and turned the engine with the plugs out and marked TDC with a file mark on the wire. I then marked 5/16" BTDC and made another mark. When I turned the engine over I thought I would see the points start to open later than 5/16" but it looked like they were strating to open at about 1/2" BTDC  *dunno*
If the timing had slipped I would have expected it to make the timing more retarded rather than advanced as the mag would be lagging the engine - can't see how slipping would advance it.
I can move the manual A/R and get the correct timing within the range of the lever so it should have run ok when I fiddled with it.
It would explain the spit back if it were too advanced but when I manually retarded it when running I got backfires.

Any ideas?

Jim
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 03.02. 2011 17:20
Another update.
Thought I would take the carb off and have another look. I know this should probably go in the carb section but may be part of the problem on this thread.
Plenty of muck in the banjo filter due to the ongoing process with the petseal dissolving so I cleaned that.
Pilot jet was partially blocked so cleaned that. Main jet looked clear.
Carb settings (389 stamped M5 on the flange):
Pilot jet 30
Main jet 106
Needle D set second notch from the top (was third before I started tinkering but plugs were sooty - now ok but bluing pipes)
Slide 389-3 1/2
Pilot screw one turn out.
Do these sound OK?
Another thing I noticed was that hole in the drip tray measured 1.135" but the carb bore measured 1.175" so is 40 thou undersize so will need some work with a file.
There is also no air filter just a gauze - covered bellmouth.

Jim

Title: Re: Spit back through carb and backfires
Post by: muskrat on 03.02. 2011 19:29
The plot thickens !
 Might be time to clean out all the petseal from the tank. Try acetone or tetra-ethel-keatone (just woke up).
 Timing will slip in the advanced direction. The pinion goes anti-clockwise, tightening the nut goes clockwise.
 Use a vernier or dial indicator to get a more accurate measurement,
 I have made a tool to find the timing position. Once you get the piston at the right height. Knock out the porcalin part of an old spark plug, run a 3/8 tap down the hole. Take the washer off the plug and screw into the spark plug hole. Now screw in a long bolt till it just touches the piston. I use red loctite so it sets there. Now every time you want to check the timing just screw in the tool and bring the piston up to touch it (gently).
Cheers
Title: Re: Spit back through carb and backfires
Post by: nigeldtr on 04.02. 2011 07:00
Jim,

Just a thought, if you are going to do the timing, perhaps change it by 180 degrees on the mag drive and then swap the leads over, see if the misfire changes side?

Don't really want to mention it but might be worth a quick check of the valve timing.

Good luck

Nigel
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 04.02. 2011 10:04
G'Day Muskrat

Great idea with the "spark plug timing tool". It is starting to make some sense if a slack bolt on the mag gear wheel causes the timing to slip to advance direction. I think all the chunks of Petseal are now out of the tank and I have filled up with ethanol free avgas. I think I will have to sort out the tank soon as you say. In the UK with our nanny state and crazy health and safety laws, it is difficult to buy any chemicals like acetone or TEK which could injure you if you accidentally make a cup of tea out of them so I may have to get a cleaning kit with the POR15 lining kit which I believe is ethanol proof.
Nigel, I think that moving the timing by 180 degrees could well be the next step if I have to take the timing cover off but rather than change too much in one go I will probably start by resetting the timing as it is and get the points gap down to 12 thou where it should be.
Before I put the bike on the lift (when it finally arrives) I think I may put the needle back to where it was (three from the top), put the carb back on and see what she runs like after having cleaned out the pilot jet. It may be that she was running weak and advanced which may have contributed to the blued exhaust - bear in mind I had to wind up the throttle stop to keep it running so may have been running on the main jet alone.


Jim
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 04.02. 2011 12:44
An update on my current woes.

I inspected the carb this morning and it looks like it is jetted wrong (see related post http://www.a7a10.net/forum/index.php/topic,3754.msg25867/topicseen.html#msg25867 It has a 240 main jet instead of a 420. The needle may be wrong - it has a D stamped on it and the alcohol version is D2. The slide is a 389/3 1/2 and my service sheet 216 says it should be a 389/3. It's amazing it ran at all?
I think I will fix this and try starting before I go any further.

Jim
Title: Re: Spit back through carb and backfires
Post by: wilko on 04.02. 2011 15:31
The "D" is right for a 389 and if you want to save some dough just file tha bottom of your slide down a tad (1/32nth) and try that then maybe a little more. The mainjet won't affect anything until it's wide open.
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 05.02. 2011 11:06
The bike lift has finally arrived so I am going out to the workshop to have a tidy up to make some space, unpack it and get the bike up onto it.
My can of carb cleaner, gasket set and a new pilot jet has just arrived so I am going to strip the carb down and clean out any congealed fuel residue and petseal.
My neighbour who is a Manx Norton specialist reckons that the mag is duff (probably the capacitor or a dodgy slip ring tracking to earth) having heard the bike running with the backfires and spit backs from three gardens away last Saturday. He was bang on when I had problems with a duff rotor arm on my Wolseley last year. He recons mags so I will take it off and get it to him.
I will keep this thread running as it will be good for other members to know the final outcome and to identify what the problem was.
Up to now I have identified three problems:
Partially blocked pilot jet
Undersize main jet
Timing too far advanced 1/2" ish BTDC on full advance.


Jim
Title: Re: Spit back through carb and backfires
Post by: wilko on 05.02. 2011 21:56
As an experiment, you could wire a car condensor into the cutout wire on the end of your mag. An old get you home trick! I'm still skeptical.
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 05.02. 2011 22:34
Hi Wilko

That would work if there was an intermittent open circuit on the capacitor but my mate the specialist reckons they usually go intermittent short circuit producing weak and random sparks which would explain the spit backs and backfires - almost like a second set of points opening randomly I would imagine. It has always misfired and run on one occasionally ever since I bought it and is difficult to start when hot - all of which points towards a mag problem. I may ask him if he can put it on his test rig before he strips it just out of interest - he normally runs them at 3000 RPM for three hours with some test gear hooked up to measure the spark strength.
I have resigned myself to a mag rebuild and with the superior insulation materials now available it should outlast me and also rule out ignition problems if the fault remains.
The other thing is that I want to get the bugger running before the sun comes out and we get our usual couple of decent weeks which we call summer.

Jim
Title: Re: Spit back through carb and backfires
Post by: starfield on 07.02. 2011 21:13
Hi,

Not sure if you have resolved your problems, if so ignore the following though its an interesting tale.

Just like you, I have been wrestling with similar problems on my 52 Star Twin - running on one and spitting back. In the end the spitting back proved to be primarily caused by tappets that had been allowed to close up. Resetting them initially seemed to cure the problem. Then the LH inlet valve stuck open and bent a pushrod, almost certainly caused by the carbon build up on the permanently open inlet valve. Naively I though the problems would go away after I stripped the head and did all the valves. Indeed the spitting back was cured but the ticking over on one cylinder and the hiccupping at lows revs remained causing a few hairy moments.

Interestingly, the bike would tick over perfectly as a 250 single on either cylinder so proving which was at fault was difficult. As the carb had already been well sorted by the previous owner, I took the plunge and had the mag serviced. What a good decision that turned out to be. There were numerous issues which were sorted out for a very reasonable cost:
1. The slip ring was badly worn and was probably the major culprit, under test it sparked like a catherine wheel.
2. The pickup brushes were wrong and may have contributed to 1.
3. The oil seal had hardened and was beginning to let oil into the armature
4 The cam ring housing was badly worn and had to be shimmed.

The last point was interesting. My mag had been converted to auto advance and the cam ring had of course been locked by grub screws. The wear had allowed the grub screws to push the cam ring off centre so the points gap differed on each side. I knew there was a problem as I could never get the timing set properly and had to equalize with 8.5mm on one side and 11mm on the other  but had assumed a worn cam ring and did not notice the points gap changing. I am not sure how such wear would affect a manual a/r situation but it cannot help and shimming is probably not an option.

Fitted the new mag, timing now identical on both sides and first kick start. No more ticking over on one, no hiccuping and no spitting back. Totally different bike. Previous owner took it for a ride today and now wishes he had not sold it. Minor moment of panic when it went on reserve on the way home and stalled at a roundabout just to shake my over confidence.

If you still have problems, maybe some of the above helps. I used the guy down at Langton Matravers who advertises in the VMCC mag. He came well recommended and I can see why. Thorough checkover, only replaced that which needed replacing and back in a week looking like new. I hasten to add that I  have no association with the business, just a satisfied customer.

Reminds me of the old story that 90% of carb problems are electrical. For the relatively small outlay compared to the value of the bike I would think a mag checkup by a specialist a good insurance.

regards

Peter
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 08.02. 2011 13:05
Hi Peter

A very interesting story. My engine has only done about 500 miles since rebuild so probably does not have the same problems with carbon build up but could possibly have tight valve guides I suppose.
I think my problem is mag related so I removed it today and will get it rebuilt by a friend of mine who rebuilds Manx Nortons.
The timing was just over 1/2" BTDC and the points gap was 15 thou though even on each side so needed retiming. The pinion was tight on the taper so does not look like it had slipped.
At least when the mag is back I can rule out ignition problems and stop chasing red herrings caused by intermittent random sparks and with the modern materials now used the mag will probably last longer than I will.

Jim
Title: Re: Spit back through carb and backfires
Post by: Brian on 08.02. 2011 13:22
Jim when you say your friend will rebuild it is he a magneto specialist? I am not trying to suggest he doesnt mean well but dismantling it and cleaning, new bearings etc will not cure it if its at fault. It has to be rewound and a new condensor fitted plus remagnetised as well as the usual bearings etc.

Unless your friend does this I would suggest you send it to a specialist magneto repairer who comes highly recommended. I am sure others on the forum can recommend someone in your area.

Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 08.02. 2011 14:34
Hi Brian

Thanks for the good advice. My friend is a first class engineer and I have every confidence in him. He tells me that he was the first to carry out the roller bearing conversion on our bikes many years back. He also works on piston aircraft engines which use mags extensively. He has all the necessary test rig for the mags and although the Manx Norton uses a BTH, he has rebuilt many Lucas mags. He sends off the armatures to be rewound but does the rest himself. He can only remagnetise BTH but knows a guy locally who can remagnetise Lucas mags. He does not advertise his mag rebuilding services as he is always snowed under with work on the Nortons. I am going to ask him if he will put it on his test rig before he strips it just out of interest although he is fairly certain it is a capacitor problem but while it is out it may be a good idea to get it rewound and remagnetised if necessary.

Thanks again

Jim

Title: Re: Spit back through carb and backfires
Post by: Brian on 08.02. 2011 22:24
Sounds like your friend knows what he is doing then, thats good.

The reason I brought it up is I have come across people before who advertise themselves as magneto specialists and who can "fully recondition" your mag but in reality all they do is dismantle them and clean them,none of the stuff that matters.

I have a bit of a thing about magneto's, whenever I buy a bike, even if it has had the maggy rebuilt and there are receipts I take the maggy off and get it rewound and a new condensor.

If the engine is worn the bike will run but will rattle and probably blow smoke.
If the carby is worn or not jetted correctly the bike will run but maybe too rich or too lean.
If the maggy is crook, the bike will not run.

The cost of a maggy overhaul is not a lot compared to the value of the bike and its nice to have bikes that start first or second kick every time, this wont happen with a crook maggy.
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 09.02. 2011 15:10
Hi Brian

Absolutely agree. I think the mag is probably the most important component and when they go intermittent they produce random running faults and it is very easy to go off on tangents looking for non existent faults elsewhere. When you look at the process involved in rewinding it is almost impossible to do at home and with the amount of labour involved I think the cost is reasonable. It looks like it is at least a day's work and by way of comparison, the going rate here for electricians, plumbers and plasterers is at least £200 per day.
The only downside is that there are few people who know what they are doing when rebuilding mags so if you want the best you have to look around and be patient as they normally have a big queue of work.

I took the mag off yesterday. I had to get and extractor as it is manual A/R so did not have the built-in puller on the pinion. The HT pickup ring was in a shocking state (sorry no pun intended) with gouges all the way round it and it looked like the brass section had a sort of clear lacquer over it - possibly something the PO had sprayed in there. I really had to dig the meter leads in and scratch the surface to get a reading. I got about 0.4 ohms on the primary and 40-90k on the secondary and the readings were all over the place so it looks like there is a high resistance somewhere.

I dropped it off this morning and I am looking forward to getting it back so I can finally get the bike running as she should.

Jim

Title: Re: Spit back through carb and backfires
Post by: trevinoz on 09.02. 2011 20:23
Jim,
         It is difficult to get an accurate resistance measurement of the secondary winding between the slip ring and earth because the secondary wire end is loosely pushed into the slip ring without a positive connection.
I suspect that there is always a bit of an air gap and the spark jumps it.
         Trev.
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 09.02. 2011 22:43
Hi Trev

That explains it. It seemed that when I pushed hard on the probe when it was on the slip ring contact I got variable readings - may have been that the slight movement made partial contact.

Jim
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 05.03. 2011 18:18
An update on my troubles.
Still waiting for the mag to come back. Apparently the guy that my mate uses to rewind the armatures for his Manx Nortons does not do many but is very good and has been doing it for ages so worth waiting for. One interesting point is that he uses two capacitors mounted 180 degrees apart to even out the balancing on the armature - never thought of that one!
I have bought an SRM timing disc, mounting nut and piston stop to get the timing spot on - 30.5 seems to be the best setting for a SR. Once set up and checked with a strobe I will make an adjustable piston stop and lock it off at the right setting for future use.
I am also replacing the existing narrow V belt drive on the dynamo fitted by the PO for the 15mm toothed belt version from Mike at Dynamo Regulators. It spins 20% faster than the SRM version as Mike has managed the squeeze an extra tooth on the drive pulley. When I spoke to Mike earlier this week he says they should be ready in a week or two and he has taken on board comments from the forum and is including a couple of tapped holes on the drive pulley to allow removal.

Jim
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 20.03. 2011 10:11
Hi all

My mag came back yesterday so I am off to the workshop today to fit it back on and re-time the bike with the aid of my new SRM piston stop and timing disc.
My new dynamo belt drive kit is on its way from Mike at Dynamo Regulators to replace the current narrow V belt.
The plan is to get the bike running by tonight and fit the belt drive when it arrives tomorrow.
Hopefully I will then be mobile as both cars are off the road at the moment and I am relying on Tesco home delivery for essentials (fags, milk and bread *smile*)
I will report progress this evening.

Jim
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 20.03. 2011 13:13
In for a coffee.

I am impatient to see if the mag has cured the problems so rather than take off the left side exhaust, footrest and drive side cover and use the timing disc I manufactured a timing aid from a fence rail bracket using a large hammer and zip tied it to the barrels. using a piece of fence wire I filed some marks in it - TDC and 5/16 BTDC and I have now refitted the mag, carb and tank (with some new avgas inside) and it is ready to come off the ramp and see if she starts.
Fingers crossed!

More later.

Jim

Title: Re: Spit back through carb and backfires SHE RIDES!!!!!!
Post by: lawnmowerman on 20.03. 2011 14:15
SHE RIDES  WOOOOHOOOOO !!!!!!!!!!!!

It is a lovely day here in the Kent countryside - the sun is shining, the cuckoo and the woodpecker in the woods - the baaing of the spring lambs but nothing quite surpasses the sound of a 650 BSA parallel twin through a pair of goldies!

It took a fair few kicks to get her into life after a short burst on the second kick and she ran on one and gave a few spit backs but when she warmed up she did not miss a beat. I need to check the mixture and play around with the pilot jet adjustment but it all looks very promising. Although I cleaned the plugs they were the old ones I was using with the dodgy mag so may have needed to be run and heated up to burn off any residual crud.

I guess the moral of the story is to make sure that your mag is ok before chasing any red herrings - a dodgy mag gives all sorts of strange symptoms depending on temperature, humidity, strength of your right leg etc and mags do die of old age - even if not used.

Jim

Title: Re: Spit back through carb and backfires
Post by: Stu55Flash on 20.03. 2011 16:56
Well done Jim:

The other motto that comes to mind is if at first you don't succeed try and try again. I;ve just got mine running after a mag armature rewind. Wouldn't start at first yesterday so retired to pub to think about it. Came back and swapped the plug leads around and she fired first kick! Some how the slip ring was 180 out from the last time! The weather up here is rubbish so I'll have to wait until next weekend to take her out any distance.

Stu
 
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 20.03. 2011 17:07
Hi Stu

It's amazing how a pint (or three) stimulates the brain cells. I was wondering which way round the leads went so I set the points with the heel on the bottom cam section as per the service sheets but nowhere does it tell you which way round the leads go. Fortunately I had some old pics of the engine so copied that arrangement. It looks like spring has arrived here at last so she will be off for an MOT next week - just have the two cars to fix now.............. At least I am mobile now and I can get down to Tesco  *smile*

Jim
Title: Re: Spit back through carb and backfires
Post by: bsa-bill on 20.03. 2011 17:11
Well done - result
Title: Re: Spit back through carb and backfires
Post by: lawnmowerman on 22.03. 2011 15:43
Looks like the saga is now over - she passed her MOT today so I went for a ride in the sunshine to the local bikers cafe to celebrate. She did not miss a beat and feels like a different bike. One strange thing was that I got and advisory for the headlamp glass which dips to the right instead of the left. Soon fixed by the helpul MOT guy with a bit of tape on the relevent area of the lens  *smile*
I am going to have a look on Ebay for a UK lens.

I will start putting some miles on her now to run-in the engine as it only did 150 miles last year after a full rebuild.

Jim
Title: Re: Spit back through carb and backfires
Post by: BSA500 on 23.03. 2011 13:23
Where abouts in Kent. I took my A7 for a buzz yesterday around Tonbridge/East Peckham.
Andy
Title: Re: Spit back through carb and backfires
Post by: Colsbeeza on 23.05. 2017 01:45
I have been absorbing this topic with interest, as I have just completed a 33-year restoration of my 1961 Golden Flash.
I fitted the tank, knee grips and badges last week. She looks beautiful.! Well- I did say LOOKS!
After a complete rebuild of the motor about 20 years ago as a fairly inexperienced mug, I got a local British bike enthusiast bloke named Phil  to strip the engine down and check it over. Fortunately, he found nothing wrong except that I had the LHS conrod round the wrong way. He started it in October 2015 and it ran sweetly on the stand, except for some smoke from the LHS cylinder which went away after 30 or so seconds. Certainly no back-spitting.
It has taken me 18 months on and off to get the wiring done - much head scratching.
Now to startup this week.
It started second kick, but would not run for more than 10 seconds due to so much back spitting that it would not pick up engine revs.
I have sofar re-checked the timing and found that the points on RHS opened about 18mm BTDC. Ahah thinks I.!  Knowing Phil's high work standards, I suspect that the ATD slipped on the taper. It came off fairly easily but I think it was tight enough!

Reset to  5/8" BTDC after much reading, and although it started immediately, the spitting was exactly the same.
The magneto was overhauled about 25 years ago which Phil thought was a bit long, so I had it re-tested last September 2015 by a well-recommended Sydney magneto specialist. He found a couple of minor problems and re-magnetised it, although he told me that the bloke who rewound it back then had had a few failures, so to be wary.
My logical next step is to strip and clean the carburettor, as there was some fuel left in it for a few months after the October 2015 startup before I got around to draining it. My first suspicion is a blocked pilot system.
If that doesn't work, I may be back to you blokes for advice.
Colin
Title: Re: Spit back through carb and backfires
Post by: Colsbeeza on 23.05. 2017 02:06
To intercept any questions about my resetting the timing, I did take a short cut, being reluctant to remove the LHS drive cover.
I first checked the points setting. The RHS corresponded with the points fibre pad being at the bottom of the camring. It was set at 12-13 thou, closer to 13. The LHS was closer to 8-9. The camring is new, so that was surprising.
I removed both plugs, refitting the RHS one about 1 thread so I could hear the hiss as the piston rose on the compression stroke. As the engine is new, it took both hands to turn the back wheel, so no spare finger to feel the compression.
After finding TDC, I used a plastic straw down the hole and marked the position with a Sharpie pen. I then backed it off, and raised the piston again to take up any gear slack until the straw reached my BTDC mark, then chocked the ATD at full advance with a wooden peg. I set the RHS so that I could just pull out a 1.5 thou feeler gauge. The heel of the points was just beginning to touch the edge point of the ramp. Previously, it had been about 1/4 inch further advanced round the camring.
I checked the points opening for the other cylinder and it was in about the same spot relative to the camring ramp. So all seems OK. Now to the shed for the carburettor clean.
Title: Re: Spit back through carb and backfires
Post by: RichardL on 23.05. 2017 04:40
There is a lot to say here, but let's start with the BTDC measuremnt. 5/8" is too much. Without knowing much about the bike, I think it is still safe to say you shuold be at 3/8" or maybe 11/32".

As for identifying compression stroke, with both plugs out and in neutral, use kickstart pedal to slowly turn over until you get pressure on your thumb (or finger), then, in 4th gear, turn rear wheel (with plugs out, of course; yes, I know you know that) to manipulate to desired BTDC.

Rather than fumbling with a feeler gauge, you would probably be better off using the fag paper method. My approach is to tear a thin strip of fag paper and put one end between the closed points and attach an alligator clip lead to the other end. Then, let the weight of the lead pull out the paper from between the points to signal when they open.

If you have the means to print a measuring graticule to scale, I would recommend printing one out with a few inches divided into 1/32" increments. Then, glue or tape this graticule to your straw. It will give you much more confidence in your measurements. Glue or otherwise fix a screw or nail in the top of your straw as a weight to be sure of positive movement. If you think it will help, rig some kind of guide that keeps the straw straight up and down.

Once you are convinced of the timing, move one of the pistons to the timing point and look into the top pickup hole of the mag (may require a mirror). If you see the brass in the slip ring, connect the top lead to the cyclinder currently on the compression stroke and vice-versa if you don't see the brass.

I'm going to step back now and let others fill in what I've missed (which will be plenty). As we say here, apologies if telling you any of this stuff is like teaching gramma how to suck eggs.

Richard L.
Title: Re: Spit back through carb and backfires
Post by: RichardL on 23.05. 2017 04:48
So, let me add in this separate post:

FIRST PLACE, BLUE RIBBON, GRAND CHAMPION of PERSEVERANCE for completing your restoration in 33 years. *yeah* *woo*  *ex* *ex* *beer*
Title: Re: Spit back through carb and backfires
Post by: RichardL on 23.05. 2017 04:52
Oops,

One more thing, for now. Don't forget to check the timing again once the pinion is tightened down.

Richard L.
Title: Re: Spit back through carb and backfires
Post by: Triton Thrasher on 23.05. 2017 05:49
Spittiing back is often a sign of weak mixture.

An air leak into the inlet manifold at the carburettor flange, is a common cause. 

Or possibly a blocked pilot jet. The Monobloc pilot jet is removeable for cleaning, but may be stuck.
Title: Re: Spit back through carb and backfires
Post by: Black Sheep on 23.05. 2017 06:53
If you don't need the choke for a cold start you are running way too rich on the first 1/8 throttle opening. Either your pilot jet is too big (how many turns out is your pilot air screw?) or the slide cutaway need to go up a number. What's your cutaway? 3, 3 1/2?  Either that or your carb float is not seating properly and the carb is flooding. As for spitting back on the move, that's a sure sign of weakness. Drop the needle a notch and see how that goes. What main jet do you have? It strikes me that the P/O just chucked a carb on with no regard to the actual settings required.
Most unlikely to be a sticking valve. 
Title: Re: Spit back through carb and backfires
Post by: Triton Thrasher on 23.05. 2017 08:56
As for spitting back on the move, that's a sure sign of weakness. Drop the needle a notch

Raise the needle to enrich!

Although if it won't run for 10 seconds, the needle taper probably isn't coming into play at these small throttle openings.
Title: Re: Spit back through carb and backfires
Post by: Colsbeeza on 23.05. 2017 09:22
Richard, Thanks for the Congratulations.!. I did a few bikes in the interim, got involved in Lilac Flat Twins and restored a 1965 Model 500 Magnum. I have already washed my mouth out with soap in case that is suggested.
I like the alligator clip idea, which would only work with fag paper. I used to use fag paper, but I didn't have any at the time, gave up the fags years ago. I read that fag paper was about 2 thou, but would work better with a clip attached. I just realised I said 5/8" BTDC. I meant 5/16" which is about 30-32 Degrees. This is 8mm. I did set at at 5/16". Certainly no sign yet of backfiring through exhaust, but it is early days. The straw did tilt a bit, which made it a bit harder, so will make up a guide next try. Yes, I had plugs out, 4th gear, still took two hands on the back wheel.
I didn't know about the idea of looking through the mag pickup.I'll do that next time. For the moment, it should be close enough to sort out the spitting if it is the carb. I'll clean that first and them move on.
Titon Thrasher - As it ran well 18 months ago, I did not recheck the carb bolt tightness, assuming that nothing would have changed. An obvious check I need to do, as the gasket (or is it an O-ring?)may have dried out and shrunk a bit. The pilot system is a more obvious check which I will do. I have to get the carb off anyway.
Black Sheep - It is fitted with a choke, as I imported the bike from Sussex in 1984 from Andy Tiernan when he was first starting out. They generally were not fitted in Australia, and I have checked that it is secured fully open each time I have started it. I'll probably end up removing it, but in the meantime it may be useful to check Main Jet size on the road. All the settings and sizes are as per the specs for this model. The carb was in exceptional nick when I received it, I suspect it was a recent replacement before the bike died way back then. I just cleaned it thoroughly back then. It ran very nicely in October 2015, so I will go through the cleaning first before chasing other possibilities. My first goal is to get it to start and run for a minute or two without konking out due to spitting at low revs.
Cheers
Title: Re: Spit back through carb and backfires
Post by: chaterlea25 on 23.05. 2017 10:11
Hi,
Problem sounds like stale fuel to me *????*

John
Title: Re: Spit back through carb and backfires
Post by: RichardL on 23.05. 2017 10:33
Col,

This is a link to an old post of mine showing my graticule tube with guide/indicator. http://www.a7a10.net/forum/index.php?topic=8301.msg58614#msg58614

All well and good. I love to talk (too much) about A10's and share my ideas, which are not always (or, "not often") original, and I am miles (figuratively and literally) from the top mechanics here. (Also, I use too many parentheses.)

Richard L.

Title: Re: Spit back through carb and backfires
Post by: Triton Thrasher on 23.05. 2017 11:01
Someone mentioned looking at the brass segment in the slip ring.

That reminds me- a dirty slip ring can cause very erratic running.
Title: Re: Spit back through carb and backfires
Post by: Colsbeeza on 18.09. 2017 11:30
Hi Gents,
I finally got the 1960 BSA Golden Flash running properly today. I went through everything, learnt a lot more than I ever knew. I was quite satisfied with the Carburettor, valve clearances, etc, so had to suspect the magneto. I did learn that a #25 pilot jet is 16 thou, and a #30 jet is 18 thou.
To recap, I had the magneto rewound etc about 25 years ago, and remagnetised and bench-tested in July 2015 as it had been sitting so long. It was firing properly when the motor was completed in October 2015. I spent a very intermittent 18 months wiring, indicators etc etc, and when trying to start it early this year, it would not fire. On the last attempt, the left exhaust got near white hot. At that point I had had enough. I sent the magneto down to Sydney for checking to the same bloke who bench-tested it in 2015. After waiting 2 months for the queue to shorten, he found about a 9 degree difference between cylinders, a bent armature shaft, replaced the slip ring and points, and took all the slop out of the points pivot pin by bushing it. How it got like this in 18 months sitting idle cannot be explained.  Electrically it tested OK.
I got it back on the bike today. It sounds very nice indeed.!
I set the timing to about 31.5 deg BTDC, and it started and ran properly, although plenty of smoke from the LHS pipe. I'll work on that later. Slight occasional spit, but I have a #25 pilot jet and as yet untuned.
I can now get on with the fettling for the road, and I do hope the smoke goes away.
Just goes to show that the old adage that "most carburettor problems are the magneto" is true.
Thanks all and Cheers
Colin
Title: Re: Spit back through carb and backfires
Post by: TimK on 18.09. 2017 23:02
Hi everyone.

I use a variation of RichardL's straw:

Quote
Glue or otherwise fix a screw or nail in the top of your straw as a weight to be sure of positive movement. If you think it will help, rig some kind of guide that keeps the straw straight up and down.

Instead of a straw I use a kebab stick, which is stiffer and less likely to bend. I attach it tightly (so it can be pushed up, but won't drop down) to the head steady with a small cable tie. This way it's pushed up to TDC, holds that position and can be marked against the top surface of the head steady to provide an accurate BTDC measurement if it's pushed back down before the engine is rotated to the next firing stroke on that cylinder.

cheers

Tim
Title: Re: Spit back through carb and backfires
Post by: Black Sheep on 19.09. 2017 07:06
And to think I wasted a couple of quid on a tdc gauge. The shame.
Title: Re: Spit back through carb and backfires
Post by: mugwump on 19.09. 2017 12:12
A sometimes overlooked cause of spitting through the carb can be caused by weak a/r springs or very worn a/r weights. This can cause a momentary over advancing on blipping the throttle. Auto a/r that is.
Title: Re: Spit back through carb and backfires
Post by: duTch on 19.09. 2017 13:11

 
Quote
Just goes to show that the old adage that "most carburettor problems are the magneto" is true.

 It just occurred to me that I had fairly much the same issue a few months ago; had the Maggie rebuilt mid-90's, was told it'd outlast me... *eek* but not so. Was running fine-ish, but then just died on me one day. I ginned around for a coupla weeks with carb and spark and beer, but gave in and gave it to the helicopter magneto guy and is now U-beaut