I run a PNP on my 1990 with 320cc injectors. It runs perfectly once its warm, and only a slight hesitation from cold.

About 25% of the time when I'm starting I get a loud pop noise from the engine, it sounds like it's backfiring. When it happens it also resets my radio.

Any idea on what it could be?

 

For whatever reason, the MSPNP fires a spark at startup. I know Ben has his fuel pump set to prime a few secs after key on for this reason.

 

Set it to "Fire Priming Pulse" "After 2 secs" as shown here. It will still spark at startup, thats just the way it is, but it won't fire the injectors immediately with key-on anymore. You should be giving your wideband a second to warmup anyway.

 

It pops half way through cranking, no as soon as I turn the key. It this still the fix?

 

If it's popping while cranking that you're injecting too much fuel. Lean out your cranking PWs.

 

I had to set my cranking pulsewidth to fixed and enter "0.1" to eliminate this. Because my throttled area includes the crossover pipe (throttle body is relocated to before supercharger), it would blow off the crossover pipe and smash things. I killed 4 thermofans and a radiator, so there's a lot of pressure in that pop!

 

when were the injectors last tested? check each plug and see if theres excess fuel on them. i believe paul had a bad injector and he got a pop everytime he started the car. installation of new injectors fixed that problem.