Before you tear it down, try this: Dump a glass of water down the carburator while the engine is running. You could have a Carbon Knock. Dump it in slowly and rev it some so it doesn't stall.
My '66 305 V-6 was famous for building up carbon. A glass of water later and it was cured. Also had a 350 V-8 that had a carbon problem. Had to use the water trick on that engine a few times during the 7 years I had it on the road. There is no telling how many engines out there got thrown away because of a simple carbon knock.
Bill
