gm 604 engine/ electrical trouble

[lpd1]

im having a issue with my gm 604. my motor is poping and back firing very loud at idle out of both sides of my headers. but at quarter throttle and above motor dose not back fire and runs really good. i acually changed motors and put my other motor in thinking it was a issue with the engine its self and it still doesit. i reused the same carb, distrib, and plug wires. i put the other motor in and it was doing the same thing. i have checked the timing several times, now i have even changed distributor, plug wires, coil, took the carb completly apart cleaned and blew air through all the holes and re assembled the carb and it still does it. the only thing i have not changed completly is the msd box which is a msd 7al. i always thought they either work or they dont but at this time im not sure what to do now. i checked the wiring harness coming from the msd box to the distributor and the purple goes to green and green goes to purple? its a rocket wiring harness.

[brad hibbard]

did you check rotor phasing?-----it sounds as if you are wired properly but the only way to know for sure is to actually check that the rotor is in the correct position

you stated that you did change the distributor so we can reasonably rule out the pick-up coil itself which can cause this condition

next would be the wiring itself-------the pick-up coil leads or (mag lead) should be isolated from any other wires and have quality connections----also the shielding insulation is very important on those leads, any cracks or splits can be a problem

after that I would check and add grounds-----with the powder coated chassis used today, grounds can be an issue

Brad