All the above are good suggestions, one more for you.Timing chain.chev sb use a nylon toothed cam gear, very common to have them wear and jump a tooth.Easy to check, remove dist cap, hand rotate eng clockwise till rotor moves, then rotate eng counter clock wise. If eng turns any significant amount before rotor turns you have timing chain issues.crank and dist should turn together at the same time. Also set timing mark on dampner to TDC no 1 cyl and dist rotor should point to no1 wire on cap.if timing chain has been done before and is a steel cam gear or a roller setuo then this isnt the problem.
