I recently acquired a 2004 model Murray rider with the afore mentioned engine.The problem is this, when idleing it runs snooth, advance the power and it backfires through the carb. I checked and changed the flywheel key( had the wrong key installed and was partially sheared), cleaned the carb, changed the plug, re-set coil to flywheel gap. I have been reading here and suspect the valves need adjusting. Anyone with other suggestions?