I would say your revs are too high. Is the engine ticking over or is it reving quite high. does it have a rev counter what is it reving out when on stand. Should be just under 1000rpm.
If its any higher you need to back off the revs. screw on side of carb does this turn in to increase revs and out to decrease. quarter of a turn at a time until revs are just under 1000rpm
bb
February 2010