Might be running lean and have detonation problem (pinging). Excessive carbon buildup in the combustion chamber can also cause this. One way you can "clean" the combustion chambers some is to spray some water into the carb while running it at like 3k rpm using an old windex bottle. The water turns to steam inside the engine (much like what happens when you blow a head gasket). Obviously too much water and you risk hydro locking and blowing the motor, too little and nothing is happening. GM used to sell this stuff called top cylinder clean that you would pour into the carburetor of the older V8's while running, but then those were non-catalyst cars and sometimes you could actually see chunks of carbon flying out the exhaust pipes. More then likely the porcelain chunks are sitting down in your cat converter now.