Yep, ditto on valve timing... had a couple circle track engines do that to me.
Also had a guy bring me a circle track SBC that would only idle fair, but went crazy on throttle. I tried EVERTYHING... litterally, until tearing the engine down to find that he had put the top timing gear drive sprocket on backwards, and the timing was out 180 degrees. Turns out his top gear had a mark on the back side also, and he didn't notice it was flipped around backwards and he was looking at the wrong mark to time it.
YES, it did run without hitting any rotating parts while 180 degrees out... he got real lucky... :shock:
I talked him out of using the gear drive for stock class circle track racing, and we put in a chain drive correctly timed, and it runs like a champ with no other changes.
Also ditto on the carb problem. I fought with a rebuilt carb that was doing the same thing. Put on a brand new carb and it cleared right up.