Crazy Backfiring

[USMC302]

IMO the heads are not the problem. Are any of those "302 experts" ASE certified mechanics that set the valves? No offense, just curious. IMO it's def valve train related from what you describe, if it's not the rockers/push rod length, you need to check the timing gears and make double sure they are installed to 0 and not advanced or retarded. I have friends use the timing gears vise chain before with no issues, as long as it was installed correctly...You hear backfiring through the intake because the intake valves are open and/or not all the way closed when the combustion chamber ignites, which could point to a degreed cam, wrong cam, incorrectly installed timing chain/gears, rockers out of adjustment, length of pushrods, so on....Your valves springs are fine if they are double springs or upgrades for a 302. What are CGM heads? The back pressure should be fine with mufflers...you are running mufflers right? Are you running stock hyd roller lifters?