zoso1

Not sure yet, I stripped down th EXUP valvejust in case but that seemed to be working ok. Nearly got the carbs stripped when the missus arrived home with the kids so progress from the on was nil! Thanks for the suggestions

Yes all plugs tight with correct leads on, they were changed middle of last year and the problem only started a couple of weeks ago. all sorted after winter carbs cleaned, new fuel etc.

sorry I don't understand?

My thunderace has started chugging badly at very low revs 1500-2500 (only when warm), it has new plugs with correct spark gap etc. once above these revs it runs smoothly with no probs. Any ideas ?