The BSA A7-A10 Forum

Technical => Amal, Carburation => Topic started by: owain on 08.09. 2017 15:39

Title: Concentric running rich on A10 plunger
Post by: owain on 08.09. 2017 15:39
I've just fitted a 930 concentric carb onto my a10 plunger to replace the leaky 289 carb. The concentric has needle position 3 (lowest). 210 main jet. 106 needle jet. The spark plugs foul very quickly (at idle) and it isn't possible to screw out the pilot screw anymore without it falling out entirely.

The carb is used (bought it at an auto jumble) and doesn't have any amal logo on it. I disassembled it and placed all the parts in a pot of citric acid (lemon juice) and left them to simmer for an hour then washed them, and used compressed air to make sure none of the channels were blocked. The pilot channel was very clear, in fact much clearer than I have found on my other concentric carbs and I'm wondering whether the pilot channel is sucking up too much petrol??

Aside form that I can only think that I need a smaller main jet (180?). Although I can't see that making much of a difference if the bike is fouling during idle. Any ideas?
Title: Re: Concentric running rich on A10 plunger
Post by: JulianS on 08.09. 2017 16:27
I think a 930 is too big for a plunger A10, a 28 mm is more appropriate.

What slide cutaway is fitted?
Title: Re: Concentric running rich on A10 plunger
Post by: Triton Thrasher on 08.09. 2017 17:28
Renew the needle jet.

Unblock the pilot jet concealed by the idle mixture adjusting screw, with a 16 thou drill or guitar string.

It is most likely that the best results will be had with the clip in the middle needle notch.

Follow the well-publicised Amal tuning procedure.
Title: Re: Concentric running rich on A10 plunger
Post by: owain on 08.09. 2017 18:51
The stamp on the slide cutaway is 928 and 2 1/2. I'll give the new needle jet a go and will clean out the pilot jet via the screw, although I'm pretty sure it was squeaky clean when I reassembled it.
Title: Re: Concentric running rich on A10 plunger
Post by: JulianS on 08.09. 2017 19:36
That slide seems too rich for an A10 - you would probably be better with a 3 1/2.
Title: Re: Concentric running rich on A10 plunger
Post by: muskrat on 08.09. 2017 20:46
I agree with the slide being too rich and 3 1/2 would be my choice also.
30mm should work OK, I have a 32mm on my A7 (as Yogi would say "hotter than your average A7, hay Boo Boo"). *eek*
Cheers
Title: Re: Concentric running rich on A10 plunger
Post by: KiwiGF on 08.09. 2017 22:45
I've just fitted a 930 concentric carb onto my a10 plunger to replace the leaky 289 carb. The concentric has needle position 3 (lowest). 210 main jet. 106 needle jet. The spark plugs foul very quickly (at idle) and it isn't possible to screw out the pilot screw anymore without it falling out entirely.

The carb is used (bought it at an auto jumble) and doesn't have any amal logo on it. I disassembled it and placed all the parts in a pot of citric acid (lemon juice) and left them to simmer for an hour then washed them, and used compressed air to make sure none of the channels were blocked. The pilot channel was very clear, in fact much clearer than I have found on my other concentric carbs and I'm wondering whether the pilot channel is sucking up too much petrol??

Aside form that I can only think that I need a smaller main jet (180?). Although I can't see that making much of a difference if the bike is fouling during idle. Any ideas?

The pilot circuit on a 930 is tricky to clean (and check its clear afterwards) if you have not done it before, I used a strand of throttle cable poked through the pilot jet behind the adjustment screw to (finally) clear mine, it was completely blocked, and bit of rubber tube and 6 fingers to block various holes to check, it took a few goes before I got it cleaned out.....

Has your concentric got a separate idle jet (in the float bowl) or is it the type where it is non changeable and behind the adjustment screw ? It could be "missing" and causing you problems.

I agree with comments ref get a new needle and jet, mainly to ensure you have compatible parts, as there are old needles and new needle jets, and they don't work together.

There are also 2 stroke and alcohol versions of concentric carbs, though I've not seen one to know the differences, and whether they can be made ok for an A10.

Have you checked the float level? It can be adjusted by warming the carb body and moving the float seat.

I'm been trying to get a 930 to work on my B31, I've decided to buy a new one from Burlen as the cost is not much more than the parts I need! (I've got wrong needle/jet combo). Unfortunately Burlen and all of the dealers are out of stock, with new ones not arriving until Oct  *pull hair out* *sleepy*

The Burlen site has a good write up on fixing up old concentrics.

Title: Re: Concentric running rich on A10 plunger
Post by: owain on 09.09. 2017 17:29
Took the carb apart again and fitted a 3 1/2 slide cutaway no big change. I did see that there is a thread in the pilot hole though. So I'm assuming that a pilot jet should be fitted? Question is...what size??
Title: Re: Concentric running rich on A10 plunger
Post by: JulianS on 09.09. 2017 17:43
The thread does not mean it needs a pilot jet screwing in. You need to check mif there is a pressed in pilot bush. REmove the air screw and if you see  brass bush it dont screw a pilot jet in the thraeded hole.

Worth reading this from Amal. Note the different spray tubes for 2 and 4 stroke and note the changes made in the needle jet and needle.

 http://amalcarb.co.uk/rebuilding-mark-1-concentric-carburetter
Title: Re: Concentric running rich on A10 plunger
Post by: owain on 09.09. 2017 19:30
It doesn't look like there is a brass pilot bush when I remove the pilot screw. The spray tube is a 4-stroke type however. So it's a little confusing. The lettering on the carb is R 930/11 (BSA B44R?). There's no AMAL logo on the carb so it could be a knock-off copy. Apparently the remoteness of the pilot jet screw caused starting and idling problems on 4 strokes with lower manifold vacuums.

I saw a post by muskrat a while back that stated .25cc pilot is a good starting point. So my current thoughts are to order a .25cc pilot screw for £4 and see if it sorts it out. Failing that...fork out the cash for a new amal  *problem*
Title: Re: Concentric running rich on A10 plunger
Post by: JulianS on 09.09. 2017 19:53
The 930/11 is definately a pre pilot bush setup.

The absence of Amal logo could simply mean that someone polished the body at some time.

I would also check that the needle jet block, needle etc are matched parts as per the Amal link above.
Title: Re: Concentric running rich on A10 plunger
Post by: BSA_54A10 on 10.09. 2017 09:27
Start by downloading this page.
All you evar need to know about concentrics
http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html (http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html)
Title: Re: Concentric running rich on A10 plunger
Post by: morris on 08.02. 2018 19:35
Did it work out in the end Owain?
Title: Re: Concentric running rich on A10 plunger
Post by: owain on 05.09. 2018 19:36
No luck with sorting out the carb issue yet. I fitted a .160 main jet and it ran fine but then the bike started to splutter and jerk really badly when the engine got warm i.e. after 30 mins riding. So I replaced the main jet with a .200 main jet but symptoms are similar.

It idles fine but if I give it more than 1/4 turn of the throttle then the bike really splutters and it sounds the engine is suffocating i.e. going to stop. I'm at a bit of a loss. I had the magneto reconditioned at APL magnetos this summer, so I assume/ hope they've done a good job and haven't returned me a duff magneto.

According to the amal tuning chart, it could be needle position (currently set in the middle notch) or a larger cut away (currently  has a 3 1/2)
Title: Re: Concentric running rich on A10 plunger
Post by: berger on 05.09. 2018 20:24
i have not been to the pub, I don't know in which order you've done things but when as a young un I got my bsa I couldn't get it to go above quarter throttle and found the timing to be retarded,,-- just a thought ;)
Title: Re: Concentric running rich on A10 plunger
Post by: muskrat on 05.09. 2018 21:16
G'day owain.
Are you sure the auto advance is working? Or if manual your moving the lever the right way? 3,1/2 slide should be OK, I wouldn't go any bigger.
Cheers
Title: Re: Concentric running rich on A10 plunger
Post by: berger on 05.09. 2018 21:39
 muskrat a friend of mine sussed that mine had been timed without the auto unit being held at full advance. in those days I knew nothing, a bit like present day, but once set up she blew the pipes out of the head and that was more *work* nowadays shes on K2FC but going back to orwains what came first ? the mag replacement or the running problem that he has now *dunno*
Title: Re: Concentric running rich on A10 plunger
Post by: kiwipom on 06.09. 2018 04:29
Yeah back in the day I just got t.d.c. On firing stroke then the fag paper release and that was that, knew nothing of wedging auto advance open, seemed to work from memory, cheers
Title: Re: Concentric running rich on A10 plunger
Post by: owain on 06.09. 2018 19:00
The original problem was that it was running super rich. I was running a .280 main jet in my concentric carb. Replaced it with a .160 and the plugs were brown and the bike started spluttering when the engine got hot. Replaced it with a .200 main jet now but the engine still splutters when I give it gas or if I'm putting the engine under load e.g. going uphill.

I've just resit the timing. I didn't set the auto-advance in fully advanced when I did it (I did do this last time though). My reasoning was that if it was too retarded before then I'll defo advance it now...

It seems that it's now advanced too much though as I'm just getting loud pops out of the exhausts!
Title: Re: Concentric running rich on A10 plunger
Post by: Greybeard on 06.09. 2018 19:59
...I'm just getting loud pops out of the exhausts!
It will do that if the timing has been swapped left to right. I know, cos I did that recently   *smile*
Title: Re: Concentric running rich on A10 plunger
Post by: RDfella on 06.09. 2018 21:09
owain - if it's too advanced, she'll kick back on the kickstarter.
Title: Re: Concentric running rich on A10 plunger
Post by: owain on 07.09. 2018 18:37
Righto, a quick update. I resit the timing. 11/32" BTDC with the auto advance sprocket wedged into full advance. Went for a ride. Same symptoms (Stuttering and spluttering with anything over 1/4 turn of the throttle). So I put the choke on so that the choke was fully lowered to richen the mixture...Cured the symptoms instantly.

So I can rule out ignition. Back to what the carb fix would be. Needle position lowest and possibly smaller cut-away I suppose.
Title: Re: Concentric running rich on A10 plunger
Post by: Triton Thrasher on 07.09. 2018 18:59
Throttle cutaway has little effect past 1/4 throttle.

To enrich the mixture at 1/4 to 3/4 throttle, raise the needle.  That is, put the clip in a lower groove in the needle.

Also check there is no water in the float bowl or anything obstructing the main jet.
Title: Re: Concentric running rich on A10 plunger
Post by: JulianS on 07.09. 2018 19:41
Are you still using the 930/11 carb?

It is quite an early one. The design of the needle, needle jet and jet holder were changed in 1969 to improve running and it could be that your carb has a mix of old and new design parts which are not interchangable.

Service sheet gives details.
Title: Re: Concentric running rich on A10 plunger
Post by: owain on 07.09. 2018 20:12
Thanks. Good advice there as well Julian. I'll check to see whether any parts are getting mixed tomorrow. Just snapped off a cheap petrol whilst trying to the remove the carb  *sad*. Another part for the list  *sad*
Title: Re: Concentric running rich on A10 plunger
Post by: owain on 08.09. 2018 13:50
Update: I removed and dismantled the carb and believe I found the problem. The needle jet had vibrated loose and was sitting free of the needle jet holder. I coated the thread in loctite and reinstalled the jet.

I guess the smart decision would be to leave the needle position in the middle notch (as it was before) and see if that solves the problem.
Title: Re: Concentric running rich on A10 plunger
Post by: Triton Thrasher on 08.09. 2018 14:28
That’s unusual, although the main jet holder sometimes falls off.

Hope you can get the needle jet off when it needs to be replaced.
Title: Concentric running rich on A10 plunger
Post by: Guy Wilson on 22.01. 2019 08:14
My main problem with running rich is that I don't ride for long enough...My regular trips are around town and rarely more than 3 or 4 miles in total a day.. I do need to address it though, but I need a longer ride and the time to do first..
Title: Re: Concentric running rich on A10 plunger
Post by: muskrat on 22.01. 2019 09:13
G'day Guy.
For short rides (3-4 miles) she would be only just getting to operating temp. Try one or two grades hotter in the plug. NGK's lower # for hotter, Champs the other way.
Cheers
Title: Concentric running rich on A10 plunger
Post by: Guy Wilson on 22.01. 2019 09:58
thanks Muskrat! good advice!
Guy