The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => Amal, Carburation => Topic started by: K1100 on 21.05. 2020 13:18
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Hi everyone. Since last week my 1954 swinging arm A10 is back on the road after 7 years standing. It was well laid up so has started easily and is running nicely. But....
I now remember the slight issue I had 7years ago when I first bought it. It starts very easily after a tickle, with no choke, and settles into a nice steady tick over and throttle response is good.
But when it is hot the revs drop slowly when I shut the throttle. They do drop, but so slowly.
I have searched this section and see that either a sticking slide or blocked pilot jet might be the cause. It has an Amal Monobloc. What should be the first and easiest thing for me to check, please?
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Can you hear the slide close with a “click?”
Classic causes are pilot mixture adjusted too weak, air leak into manifold, auto advance sloppy or sticky.
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May be nothing more than a dry or tightly routed throttle cable, but as a first step check the slide. Grit, varnish from old fuel and carbon build up are easily removed with a dose of carb cleaner.
As TT says, it should close with a definite click. While the top is off the carb.....check the slide actually has a return spring that works, and it does not bind on the choke slide. Applies to 276 and monobloc types.
Earlier post in this section details a typical problem and cure for a sticking slide.
Swarfy.
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Hi Swarfy and Thrasher. Throttle closes with solid click hot or cold. Found that the pilot jet cover nut was loose so have tightened that (after making a cutaway in the side of the drip tray so I could move a spanner).That doesn't seem to have changed anything so is carb removal to take out and clean pilot jet next do you think? The drip tray gets right in the way of the pilot jet.
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G'day K1100.
Sounds like an air leak to me (as well as above suggestions). All good cold/warm after a tickle up but when hot and the carb grows a little (more than the head) you may have a leak at the flange. A quick check is to get her hot and spray some WD/carb clean around the flange and listen for revs to drop or increase. Also spray around the top cap and cable entry.
Cheers
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Is your slide worn out?
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Do you have a choke cable? If not is the hole in the top of the carb blocked off?
Mine used to have an erratic tickover when hot, tickover revs increased randomly, cause was a worn slide, solution new burlen amal.
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Hi Kiwi GF, Muskrat and Trevinos. I’ll work through those suggestions ...thank you. I can see from the file of receipts that came with the bike that the carb was new 11,000 miles ago which I am thinking should mean it is ok inside.
Best wishes to all.
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You'd think it would and should be good - but a new 389 on mine, the slide was dead in less than that mileage and I replaced it with the better anodised version. Which IMHO ought to be the standard fitment, not a pay-more-for extra. Presumably they exist because it's well-known the cheese-metal ones aren't much good!
Mine was both 'sticky' in operation in the bore and 'rattly' when closed, which is kind of counter-intuitive, but it was what it was. (The bore was still good luckily.)
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Don't know if they still do, but Surrey Cycles used to do chrome plated slides. I bought my 389 from them and that's how it was delivered. My slide has never shown any sign of wear.
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Slide wear presumably has some dependency of the presence of an air filter and the environment you normally ride in?
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The change from instant shut-down to slow shut down occurs within 2 mins at tick over. Today I dribbled oil over the carb/manifold joint but couldn’t sense any improvement.
Tomorrow I will remove the carb and remake the joint with new gaskets, to be certain there is n9 leak there.
If the pilot jet is the issue, is it that it is not properly adjusted or that it is blocked?
Removing the carb gives me access to check it... the drip tray gets in the way at the moment. *sad2*
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Sounds like a modern car - they take ages to drop to a tickover - apparently otherwise they'd stall because of their lean burn. Makes them unpleasant to drive. In your case, sounds like a too-rich slow running mixture.
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Hi RD Fella... thank you, that’s high up on the list to check, and I am still surprised that the drip tray stops me getting any tool to the pilot jet to work on it.
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The pilot jet is not making sense to me. When I unscrewed the pilot jet retaining nut the float chamber contents drained out. The pilot jet was loose. The retaining nut screws onto the jet so I can't see how to adjust it. Wherever I set it - say 1 1/2 turns out - it will screw back in as I thread the retaining nut onto it. What am I missing?(apart from a brain)
And how is it possible to adjust tickover while the engine is running if the fuel drains out? Haynes is useless and I can't find anything online that helps.
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I might have worked this out.
The pilot jet that is in my attached pic is not adjustable. My mistake. It should be screwed right up and home, and the pilot jet retaining nut screwed tight onto it so it all stays put. If that jet was loose, that might have been causing my rev holding issue. So the petrol drip tray position doesn’t matter, because I don’t need access to the jet to adjust it... the jet just has to be screwed right up and tight and the retaining nut and fibre washer lock it in place.
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Sorted... it was the loose pilot jet and pilot jet retaining nut
I now know about 10x more about carbs than I did two days ago so thankyou to everyone who put thought into my problem and helped me to sort it.
PS I know from the paperwork with the bike that the guy I bought it from in 2012 had a few years earlier shipped it from the UK to NZ for a long touring holiday with his wife and camping gear.
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The pilot jet is not making sense to me. When I unscrewed the pilot jet retaining nut the float chamber contents drained out. The pilot jet was loose. The retaining nut screws onto the jet so I can't see how to adjust it. Wherever I set it - say 1 1/2 turns out - it will screw back in as I thread the retaining nut onto it. What am I missing?(apart from a brain)
And how is it possible to adjust tickover while the engine is running if the fuel drains out? Haynes is useless and I can't find anything online that helps.
Hi K1100
The screw you should be moving to adjust the pilot jet is the horizontal one directly above the pilot jet, this controls the amoount of air flow of air across the top of the pilot jet. The pilot jet should be screwed fully home - but not too tight as it's only brass.
Cheers
Tim
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Brass jets and Mazak Carb Bodies can have a strong affinity. I found this out the hard way simply adjusting the tickover on a Monobloc. Thought the adjuster screw was a bit tight, next thing the damn thing sheared off, effectively rendering a good carb just a bit useless. These days a smear of silicone grease on such threads solves the problem for me.
The earlier 276 carb has a much less tight thread and a locknut on the adjuster, never a problem here.
Swarfy.
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Hi Tim and Swarfy... thanks for your help. All is going well now - starts easily, tickover steady and rev hanging gone.
It was the loose pilot inside the brass nut that caused me a mental flip.. because the jet was loose, I assumed it was adjustable, whereas it was just loose because it had come undone.