Thanks, Steve. I think the pilots are stock (I'll check 'em) and the air screws are set as per the factory settings. When I got it, it had been put together without the gasket between the manifold and the cranckcase (instant gasket instead ). I fitted a new one, thinking that might cure it, but it's just the same.

I have a receipt for a full shop motor rebuild less than 1000 kms ago, but if it was them (Two Wheel Technics, Plymouth. Know anything about them?) that built it up without the manifold gasket, I don't know whether I should suspect a crank seal if the motor has just been lashed together by the YTS lad.

Cheers

Wb