need help fixing a engine/ electrical problem

[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.

[jedclampit]

I'd swap the pickup wires & re time it first and see what you got.
I'd bet its the MSD.Easy enuff to swap out and rule in/out.
The ignition switch should be changed out too, easy as it is. I'd change it out every year if running a used car.

[MasterSbilt_Racer]

Read this thread and check your setup out.

http://www.4m.net/showthread.php?258069-BEWARE-msd-ignition&highlight=polarity

[lpd1]

I'd swap the pickup wires & re time it first and see what you got.
I'd bet its the MSD.Easy enuff to swap out and rule in/out.
The ignition switch should be changed out too, easy as it is. I'd change it out every year if running a used car.

how can you swap the pick wires? cut and splice?

[jedclampit]

how can you swap the pick wires? cut and splice?

Take the terminals out of the connector, and swap positions in connector housing near the dizzy. If it has the stock small white plastic connector still, its long past due to put a good Weatherpak connector on.

NO CRIMP CONNECTIONS on a racecar for a trouble free electrical system all terminals get soldered.

Is this in the shop too or only while under a load.

BTW....maybe there is no coolant in it........J/K.

[FlatTire]

I had some broken plates/terminal side posts on a battery cause me some issues like you are describing. Detonated an engine before we figured out what was up.