1971 Slimline 250 Sherpa ,help Needed

[scotty]

Ok, guys ive now installed a new ign side crank seal, im sure the clutch side seal should be ok as 1, the bike dosen't smoke excess, 2, the bike starts 1st or 2nd kick! BUT ive still got a problem with the lower end of the rev range! the bike idles for a very short while then dies! but the revs dont pick up well as in you have to throttle it lts to keep the revs up and stop it from dying! and have to give it loads of revs to pull away! the mid and top ends good and it will really get a move on when needed! ive cleaned out the carb , jets etc! the vent pipe on the tank clear, new s/plugs gapped and fitted! fresh petrol added! the timings not been checked as i need to get a TDC gauge! ive not checked the airfilter yet nor the position of the carb slide needle these to jobs are next! im very tempted to get a brand new amal mk1 for it if i can get one jetted corectly etc! as im sure this problems due to fuel stavation somehow? the float chambers filling up ok! Help!!!!!!

[woody]

If it is the original carb and the bike has had a lot of use in its 30 odd year old life then the chances are the carb body is worn, as well as the slide and letting air past the slide. This is only noticeable on very small throttle openings, when air leaks around each side of the slide, weakening the mixture and can give symptoms similar to those you are describing. When on half throttle and above the slide is open far enough that it is pretty well clear of the venturi and therefore the effect of the worn body/slide isn't noticeable. It is also common for the engine to not shut off cleanly when the throttle is closed as the air leaking past the slide will draw fuel and make it run on a bit until it dies. If the body is worn you'll waste your money putting in a new slide as it will still let air through.

If you are serious about using the bike, buy a new Amal. You can no longer get an original type 627 27mm version but the 626 26mm will work just as well. You can pee around for ever with the old carb and you may always be compensating for some wear somewhere when trying to adjust it. It could even have had the flange overtightened at some time which may have distorted it, another place where air can get drawn in - check it against a flat surface.

If you replace the carb, I'd replace the plastic heat shield/gasket it bolts up against too.

Jetting should be no problem if you buy the carb from Miller as they will know the sizes. Not sure if bultaco UK stock the MK1 amal. You could also go for the MK1 version which has a choke lever instead of a tickler. That's the only difference, same carb apart from that.

From memory, I think jetting is 25 pilot, 106 needle, 160 main, 3.5 slide. Amal MK1 needles are all the same (2 stroke that is), there is only one type - set on middle notch.