Timing and lean condition, the alcohol racers have a valve they open to let more air in (lean) while running to make it heat up faster. Detonation could be a problem, try different advance settings. These engines where made for 260 Sonoco not the junk we have today. Basically burns at different speeds. Did anyone install a non stock cam? If so it could be installed retarded. There are so many places to look so try the IF gun first.