What will cause a 1985 Chevy Celebrity to backfire checked timing compression both good vacuum gauge indicates valve timing out but heard flat cam and burnt valves or is it a timing set?

Since the compression is good I would not expect it to be a burnt valve. It could be timing if the timing chain has excess play. I would also check the ignition module and the engine control module. We had a 1986 Chevy Celebrity that had an ignition problem that cause it to run badly and backfire. The service engine soon light was blinking but not posting a code. We changed the engine control module and it fixed the problem.