Thanks to all who helped. Heres the reasoning.

At first, i couldn't get the motor to idle below 850. Very high, i know, but with a cam and no mufflers it seemed to need that much to sit happily. But of course, that was wrong.

Reason it wouldn't sit lower, as i discovered in time since, was sticking advance weights that kept shifting, and a dodgy lead (brand new quality gear too...hmpf!). Plus slightly incorrect carby settings and a few other little bits and pieces.

After subsequently fixing ALL those, i still couldn't get it to idle....until i realised i'd been having a MAJOR brain fart. At 800 it was drawing all the fuel from the idle circuit, AND from the main, so it was fuelling up and running lke ****. In fact, at 950 rpm i could close off the mixture screw completely, and it idled on the mains!

Now i've set her, and she idles sweetly and consistantly at 600.

I can't bleieve how simple it was, BUT i had to fix many smaller faults before i could get to that.

Always the case isn't it?