mine does that but i just put it together after 20 years so don't know yet what it is. i put some 93 octane fuel in it to see if that helps. if that doesn't help, i'm going to retard my timing from 12 BTDC to 10 BTDC. it does seem like its pop and cough on moderate to heavy accel which could be detonation caused by overavanced spark or poor fuel quality. hopefully one of those works, i'll let you know.