With a correctly set up to spec Amal 276 and no air leaks, at idle, is the engine running solely on the slow running jet controlled by the air screw or is it also pulling a small amount of fuel past the clearance between the parallel part of the needle and the needle jet?
My understanding, someone will correct me if I am wrong, but most Amal carbs work in a similar fashion.
The air screw controls the fuel air mixture and screwing it in richens the mixture and out lets more air through and weakens the mixture.
Normally at tick over the throttle slide is completely closed on the inlet manifold side, the cutaway on the carb intake side may have a slight gap but does not have any influence at this stage as the slide is at the bottom and the needle similarly so. Sometimes the throttle stop is raised and the cut away can come into play.
When the throttle is partly opened it is the slide that has its effect in conjunction with the pilot system, the needle is still in the needle jet and on the parallel (fat) part of the needle, later the needle and jet and finally the main jet once the needle is high in the needle jet.
This diagram shows it best:
(https://amalcarb.co.uk/media/wysiwyg/contentimages/amal_throttle.jpg)
If the throttle valve is completely closed then the engine would be running solely on the pilot jet controlled by the air screw. However, once the throttle valve starts to open, the pilot jet is then progressively augmented via fuel drawn past the needle; essentially, past the non tapered part of the needle.
Just to clarify: normally, to make the engine idle, it takes a combination of adjusting the air screw and the throttle stop screw. This means that, in most cases when the engine is ticking over, the throttle valve is adjusted to be slightly open. Once it is open, the fuel flow from the pilot jet will be augmented by a small amount from the needle jet that flows past the non-tapered part of the needle at this point. If this did not happen, it would only be the pilot jet providing the fuel flow up to about 1/4 throttle!
Thanks for the quick response. Let me give you a bit more detail. Have tinkered on with British bikes for the last 50 years using 276 and later monobloc carbs setting them up as per the book with little problem.
This particular 276 has been rebuilt with new jets needle etc. The only deviation from standard is it has a 6/3 slide instead of a 6/4 as it is a good fit in the body was retained, perhaps this may be the issue?
When setting up the idle with a warm engine using the throttle stop and air screw i found the slowest reliable idle speed was with the air screw as good as fully closed. Am I right in thinking that fully closed the air screw should pass nothing or does it still let some air by? Hence my question as to whether fuel still passes between the needle and needle jet.
Would a one size richer slide make such a difference?
The engine responds well to the throttle with no hesitation or spitting back.
I'm not that familiar with 276 carbs but, from what I can see, there isn't a pilot jet as such but just a pilot fuel passageway. In this case, if the air screw is fully screwed in to minimise the air flow through the pilot system, then one possibility is that the pilot fuel passageway is being restricted - effectively, the pilot jet is partially blocked. The first thing to do therefore is to ensure the pilot passageways are completely clear but I am not sure how easy this is to do on a 276? On a Monobloc, it's straightforward because it has a separate pilot jet but may be more difficult on a pre-Monobloc. If it's not easy to get at the pilot passageways to clean and inspect, ultrasonic cleaning might need to be considered?
Had it completely apart twice. The smallest passage in the pilot system is 1/64 inch (0.015 inch) diameter in the jet block. Had to buy a drill to make sure clear. May well have to carry out a strip for the third time to check but not confident will find anything.
In my experience the vacuum is insufficient at idle, assuming the correct carb, to draw any significant fuel past the needle.
If the cutaway was effecting the transition from pilot to needle it would spit or cough depending if too lean or rich at transition and, again, has little if any effect on idle, assuming it is not raised too far causing transit. Unlikely.
Ignition timing can play as much a part at low engine speeds and needs to be checked as well. AA and electronic advances not appropriately retarding or tired magneto wiring, points gap, valve clearances etc.
If the carb is worn this will have a seriously detrimental effect on tick over too. Lots to check.
If the pilot passageways are clear then the problem may well be air leaking past the sides of the throttle slide. If in good condition, the slide should be a snug fit within the carb body without any obvious side play. Unfortunately, carbs wear and, although you can replace the throttle slide, you can't do much about the carb body other than to replace it.
Didn't Martin Bratby offer a service resleeving bodies? Worth checking as he's still in business.
I agree with Spaceman, you probably have a blockage in the pilot system giving you a very weak mixture. There are 2 holes in the air intake, the pilot bypass and the pilot outlet. These are either side of the slide. If you remove the brass jet block, you may be able to see that there are 2 holes within the sunken hole, one of which is drilled at an angle through the jet block to act as the pilot by-pass and the other lines up with the hole in the carb body to act as the pilot outlet. You should be able to use an airline to poke up the body and blow through the 'large' hole in the carb wall. You can check it's clear by squirting WD40 into the hole. If you then refit the jet block, you can squirt WD40 up the hole in the base of the jet block, to see if it comes out of both holes in the intake.
I had exactly the same problem, and on removing the jet block, found a piece of debris blocking the sunken hole. I blew it through and refitted the carb, knowing that it was now clear. Made no difference, so you waste time looking elsewhere. Eventually I removed the jet block, and found it blocked again. I had to repeat this exercise a dozen times, sometimes I could see the blockage, sometimes I couldn't.
I finally realised what was happening. The tank sealant had disintegrated, and each time I cleared the blockage, it just blocked up again straight away. Although I had fitted a tank adaptor with a filter and a tap with a filter, debris still got through, until I fitted an in line filter which cured the problem. Been fine ever since, but I must clean the tank out one day.
I'm not sure what holes you are measuring at 1/64", but the holes in all my carburettors are considerably smaller than this. Maybe, over the years, people have used drills to clear obstructions, but this will enlarge the holes each time they are used. I use fuse wire.
The other possibility is a bad air leak. It may be worth spraying the inlet valve with grease with the engine running to see if the engine slows down. If so, you have a worn valve guide. As already mentioned, a worn carburettor body could also require the mixture screw to be screwed right in.
Let us know how you get on.
The 1/64 (0.015) diameter hole referred to is marked in blue which is the same size as fuse wire I initially used.(//)