Backfiring

[WTRoadrunner]

Did you check the compression on every cylinder? Should be about the same give or take maybe 5 psi. Feel of the valve springs, I found 2 broken that I missed for a while. # 7 and # 8 exhaust. That will cause a pop out the exhaust. With a compression check you know what the cylinder is doing.

[WTRoadrunner]

I don't mean to be a pain...I built heads for 15 years...If the intake valve is closed when it fires...no way will it back fire through the carb and the same for the exhaust. If the timing is on. 8 before tdc is close enough, I prefer 10 with a 10 to 1 comp. I replaced carb, set valves, changed wires, plugs and even called Lunati before I ran the compression. Thats when I discovered 2 cyl that were weak causing problems. Its really simple if you stick to the basics. This is why we play with these old trucks. Good luck!!!