Author Topic: Spit back through carb and backfires  (Read 14467 times)

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Spit back through carb and backfires
« 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

1959 A10 SR
1938 Wolseley 14/60
1955 Ferguson TEF20 tractor
1965 Ferguson 135 tractor
1952 Matchless G80 rigid
1960 BMW R60
1954 Matchless G80S
1955 Ariel 500 VH
1951 Sunbeam S7DL
1960 Matchless G12 with Watsonian Monza
......and loads of lawnmowers

Too old to Rock and Roll but too young to die  (Jethro Tull 1976)

Online bsa-bill

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Re: Spit back through carb and backfires
« Reply #1 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
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

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Re: Spit back through carb and backfires
« Reply #2 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
1959 A10 SR
1938 Wolseley 14/60
1955 Ferguson TEF20 tractor
1965 Ferguson 135 tractor
1952 Matchless G80 rigid
1960 BMW R60
1954 Matchless G80S
1955 Ariel 500 VH
1951 Sunbeam S7DL
1960 Matchless G12 with Watsonian Monza
......and loads of lawnmowers

Too old to Rock and Roll but too young to die  (Jethro Tull 1976)

Offline sinbad

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Re: Spit back through carb and backfires
« Reply #3 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
Rod
1960 A10                             1998 Honda Cb 500
1953 sunbeam s7                   1988 Mz 250
1953 Nsu Consul 500 single        Jzr cx 500 Trike

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Re: Spit back through carb and backfires
« Reply #4 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
1959 A10 SR
1938 Wolseley 14/60
1955 Ferguson TEF20 tractor
1965 Ferguson 135 tractor
1952 Matchless G80 rigid
1960 BMW R60
1954 Matchless G80S
1955 Ariel 500 VH
1951 Sunbeam S7DL
1960 Matchless G12 with Watsonian Monza
......and loads of lawnmowers

Too old to Rock and Roll but too young to die  (Jethro Tull 1976)

Offline sinbad

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Re: Spit back through carb and backfires
« Reply #5 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.
Rod
1960 A10                             1998 Honda Cb 500
1953 sunbeam s7                   1988 Mz 250
1953 Nsu Consul 500 single        Jzr cx 500 Trike

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Re: Spit back through carb and backfires
« Reply #6 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
1959 A10 SR
1938 Wolseley 14/60
1955 Ferguson TEF20 tractor
1965 Ferguson 135 tractor
1952 Matchless G80 rigid
1960 BMW R60
1954 Matchless G80S
1955 Ariel 500 VH
1951 Sunbeam S7DL
1960 Matchless G12 with Watsonian Monza
......and loads of lawnmowers

Too old to Rock and Roll but too young to die  (Jethro Tull 1976)

Online bsa-bill

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Re: Spit back through carb and backfires
« Reply #7 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
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Offline cyclobutch

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Re: Spit back through carb and backfires
« Reply #8 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?
Various, including ...
'58 Iron Head Flash Bitza


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Re: Spit back through carb and backfires
« Reply #9 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
1959 A10 SR
1938 Wolseley 14/60
1955 Ferguson TEF20 tractor
1965 Ferguson 135 tractor
1952 Matchless G80 rigid
1960 BMW R60
1954 Matchless G80S
1955 Ariel 500 VH
1951 Sunbeam S7DL
1960 Matchless G12 with Watsonian Monza
......and loads of lawnmowers

Too old to Rock and Roll but too young to die  (Jethro Tull 1976)

Offline Stu55Flash

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Re: Spit back through carb and backfires
« Reply #10 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
"Keep a distance from lady "L" drivers in cars. Some are not mechanically minded, are slow to acquire road sense, an are apt to panic..." The Pitman Book of the BSA Twins.
Golden Flash Plunger 1955, Francis Barnett Falcon 67 1954, Ferguson TEA Tractor 1951. Looking for another project!

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Re: Spit back through carb and backfires
« Reply #11 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
1959 A10 SR
1938 Wolseley 14/60
1955 Ferguson TEF20 tractor
1965 Ferguson 135 tractor
1952 Matchless G80 rigid
1960 BMW R60
1954 Matchless G80S
1955 Ariel 500 VH
1951 Sunbeam S7DL
1960 Matchless G12 with Watsonian Monza
......and loads of lawnmowers

Too old to Rock and Roll but too young to die  (Jethro Tull 1976)

Offline nigeldtr

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Re: Spit back through carb and backfires
« Reply #12 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
1951 Golden Flash (engine now rebuilt) 1953 M21 a pain to start and 1961 GF that is turning into a black hole!

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Re: Spit back through carb and backfires
« Reply #13 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
1959 A10 SR
1938 Wolseley 14/60
1955 Ferguson TEF20 tractor
1965 Ferguson 135 tractor
1952 Matchless G80 rigid
1960 BMW R60
1954 Matchless G80S
1955 Ariel 500 VH
1951 Sunbeam S7DL
1960 Matchless G12 with Watsonian Monza
......and loads of lawnmowers

Too old to Rock and Roll but too young to die  (Jethro Tull 1976)

Offline nigeldtr

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Re: Spit back through carb and backfires
« Reply #14 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?
1951 Golden Flash (engine now rebuilt) 1953 M21 a pain to start and 1961 GF that is turning into a black hole!