is it back firing or just sputtering. If its backfiring More than likly its sounds to me like a weak valve spring thats not closing correctly or timing. Its possible that your distributor(or other componets associated with it ie. cap, rotor button, or igniter) is messed up so that the spark is hitting the chamber at the wrong time causing detonation. If its just sputtering it could be a bad sparkplug/sparkplug wire or same as stated above..