couple things. you need to run the 195* T stat. it makes a huge diff on fuel mix. if it thinks its cold then it will dump fuel.
secondly carbon build up. if you have a layer of carbon on the pistons it will cause detonation. its a lot like a back fire but it can go to the intake side or exhaust side. flush with GM top engine cleaner right down the TBI.
Run the engine till hot, then pour into the TBI while running and drown the engine out with the bottom 1/3 of the can. let sit for at least an hour. then restart. it will smoke like hell. run till it cleans out then take for a drive.