I think the ignition needs adjusting, there is a slight flat spot at 3500-4000 rpm which no amount of fiddling with the carb has fixed. With a strobe the timing is spot-on but it "jumps about" at the flat spot rpm, and I have been told the air gap in the pick-up may be wrong.

Can anyone shed some light on this and explain how to cure the problem ???

Does anyone know anything about setting up these engines without points ???

Any help and advice apreciated.

I think the ignition needs adjusting, there is a slight flat spot at 3500-4000 rpm which no amount of fiddling with the carb has fixed. With a strobe the timing is spot-on but it "jumps about" at the flat spot rpm, and I have been told the air gap in the pick-up may be wrong.

Can anyone shed some light on this and explain how to cure the problem ???

This might be a difficult one to solve, Firstly, have you got a haynes manual or another manual, that will tell you how to set up the ignition timing, it does however requre a dial test indicator and spark plug adapter to set it to the required 2mm BTDC. I'm a bit confused about this though because you say you checked it with a strobe and it was spot on, so in suspecting the timing you may be barking up the wrong tree. It might be a missfire at 3500/4000 rpm and that could be caused by almost anything, could be

an air leak somewhere

someting blocked or dirty in the carb

faulty CDI

faulty spark plug

worn piston rings / bore

not sure about the air gap you mention, I wouldnt have thought this is an adjustable setting, but if it is then yes maybe its even this, to what extent did you strip and clean the carb if you screw the mixture screw in and out does it have an affect on the idle speed?