Just re-read the Amal site on the effect of a the screw pilot Jet on 4-strokes, hence why they introduced a press in bush instead:
Pilot JetsWhen first introduced, all carburetters were fitted with a detachable pilot jet screwed into a threaded hole in the bottom of the carburetter body. This arrangement is retained for 2 strokes, but from 1968, was replaced for 4 strokes by a bush pressed into the gallery behind the pilot air screw.
The detachable pilot jet caused problems with some 4 Stroke engines. The remoteness of the jet from the two pilot circuit outlet holes, caused starting and idling problems on bikes with lower manifold vacuums. This could cause stalling during deceleration, due to a weak mixture. The introduction of the pilot bush moved the pilot jet nearer to the outlet holes, overcoming this problem. The bush has a flow rate, equivalent to 20cc/min. Carburetters fitted with a bush, retain the threads originally used to fit the removable pilot jet.
I presume the B40 has a low manifold vacuum ? as the engine has a low compression ratio
I have a screw-in pilot jet as its a converted 2 -stroke carb & these symptoms fit ... or sort of fit ?? The symptoms I have are:
- bike starts easily
- once warm it may idle evenly for say 20 secs or more , but will dip/flutter every now and then.
- the flutter sometimes is enough to stop the engine.
- the flutter does not have consistent pattern & seems relatively random in occurance
- Up to 1/8 turn ( opening throttle slowly then held steady) a similar pattern of occasional flutter happens, but perhaps at less frequency
- there's a bit of a popping/ringing sound from the exaust ( post combustion of fuel in the exaust ?)
- there is hesitancy and lumpiness if the throttle is twisted quicker.
- if the throttle is closed swiftly the engine can die....or sometimes the revs dip and the carb spits, then recovers or dips and recovers without spitting.
- on the road it seems more even when accelerating, but lumpy and jerky at constant revs or deceleration.
Any views on if the screw in pilot jet could be a substantial part of my problem ?(I have other reasons that may justify buying a new Concentric Premier carb. e.g. I have no O ring on mine at the manifold flange, or Stay-up float)