Sounds like carby to me ,I would try a carby off a known good running power torque engine that way you can at least rule out that side of things,id also be looking at the throttle cable that little ball on the end can be a bugger at times to seat home.....Sounds like you know your stuff and are doing the right thing eliminating one thing at a time....Good luck not that you need it. you will have it going in no time...cheers Chris
