Hi, am new to the delights of air-cooled ownership, and already my troubles have begun! Basically, she starts fine first time, the choke comes on and then off as it should, drives fine, however, if after driving i come to a stop at lights etc, she will often be inclined to stall. Have tried making the mixture richer and and raising the idle speed, which has improved things a bit, but she still stalls at times. Normally she will run stationary with no problems, but occasionally the revs will gradually die off until the engine stops. This always happens with the choke off and the engine warm. Not good! Any suggestions would be much appreciated.
But, if the weather is warm where you are, stalling can often be caused by the fuel in the carb bowl boiling due to high temps in the engine bay. This is COMPLETELY different from so-called "vapor lock".
A similar condition exists if the float needle valve doesn't seat tightly or if the valve is not screwed in tightly.
Other causes would include tiny vacuum leaks on the intake manifold (especially at the rubber booties) or at the throttle shaft.
If it's cold where you are (~45F) and damp, you could even be seeing carb icing.
Uneven idle and dying, 95% of the times is caused by improper carb adjustment or clogged / partially clogged circuits in the carb. What I do whenever the idle speed isn't steady and stalls, is to remove the carb, thoroughlly clean it anyway by completelly dismantling it (removing every jet and adj. screw, blow passages with compressed air, fit new gaskets and O-rings, lubricate properlly). It's a 30min job if you get experienced with it, or a couple of hours for a beginer if you allready have a carb rebuid kit and an air-compressor. Then reinstall the carb and readjust the idle circuit.
Procedure for Adjusting PICT carbs
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0) Before starting the engine, you have to do several pre-adjustments to ensure that the engine will run and to simplify the adjustments. Screw all the way in the two adjusting screws on the left. Do not apply excesive force when they bottom, otherwise you will deform the seat in the carb's body. [ Remark: These two screws have a small O-ring seal on them. This should be lubricated with a bit of oil before installing these screws in the carb, so that the seals won't be chewed and they wont bind when you try to adjust them. ] Then unscrew 3+1/2 turns the big screw and 5-6 turns the small screw.
1) Now start the engine and warm it up.
2) Adjust throttle butterfly so that it closses completelly. Otherwise the idle circuit won't work properlly. To do so, unscrew the throttle positioning screw all the way out, then screw in until it just touches the fast idle cam in the slower idle setting, then additional 1/4 (about) turn in. This is so that the thottle lever stops on the adj. screw and the butterfly doesn't bind on the carb's throat.
3) Adjust idle speed at about 900 rpm with the volume control screw (the big one on the left of the carb). On older carbs without a volume control screw, this setting is achieved with the thottle lever screw.
4) Adjust idle mixture with the small screw on the left. To do so, screw the mixture (small) screw in untill the engine speed begins to drop. Then back out 1+ 1/4 turn. If there is no response in engine rpm when adjusting the mixture screw, the idle jet or idle circuit passages on the carb are clogged.
5) Readjust the idle speed with the volume control (large) screw to 850-900 rpm.
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