could be a number of things 1 -being low compression due to piston damage after getting too hot.2-carburation fault, diaphrams inside worn ,needle valve leaking,H&L mixture screws incorrectly set.3-air leaks manifold, crankcase or seals.Check spark plug if it has a grey residue on it is probably aliminimum an indicator of piston damage,if petrol in cylinder carb flooding,base setting on mixture screws all the way in clockwize then out 1full turn out each plus1/16th then to 1/8of a turn see if this of any help
Dave Fix, May 2011
