Concentric running rich on A10 plunger

[owain]

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?

[JulianS]

I think a 930 is too big for a plunger A10, a 28 mm is more appropriate.

What slide cutaway is fitted?

[Triton Thrasher]

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.

[owain]

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.

[JulianS]

That slide seems too rich for an A10 - you would probably be better with a 3 1/2.

[muskrat]

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").
Cheers

[KiwiGF]

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 

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

[owain]

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??

[JulianS]

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

[owain]

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 

[JulianS]

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.

[BSA_54A10]

Start by downloading this page.
All you evar need to know about concentrics
http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html

[morris]

Did it work out in the end Owain?

[owain]

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)

[berger]

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