Originally Posted by spectre6000
The closed throttle means there is very little fuel coming from the carb, yet the engine is still turning due to the movement of the car through the transmission. The mixture in the cylinders is too lean to consistently burn, so some of it is making it to the exhaust where it is meeting the hot gasses from cylinders that did burn and igniting in your exhaust. The holes in the exhaust obviously aren't helping and may make any tuning attempts moot, but you may find turning your idle mixture screw out a quarter to half turn will rectify this to some extent.
I know all this; however the carb is perfect, it is a Jim C. rebuild and was working perfectly prior to the motor rebuild. The mixture upon negative acceleration is not too lean, it is just right. The idle mixture is also just right, and the jets are perfect for my elevation.
Once I check the valves (and adjust if necessary) and replace the exhaust, I will post up if I need any help. That's why I haven't posted - I know what to do and am not stumped, right now I am working on a rear bumper project for that rig that I am halfway through.
Hugh's guys at Safari LTD did the rebuild, and while they are pretty good, a fully smogged 60 has a buttload (tech term) of emissions subsystems that could have been reconnected incorrectly, and there are lots of places for an incorrectly connected emissions subsystem to cause backfiring as well. I have plenty of (really time consuming) experience in troubleshooting each subsystem and have been putting that off until after I check the valves and replace the exhaust.