Stock ignition + Nitrous ??

I can't remember how big of a shot you are running? What RPMs are you shifting at? Hopefully you're not shifting at the same RPMs as you do when running on just motor? You should shift a few hundered RPMs lower when on the juice. My mods are in my sig. I have a shift light that comes in real handy when drag racing, especially when using nitrous. I have a 5800 shift pill in the light when on motor. I put a lower, 5600 rpm pill in when using the nitrous. It seems to help out. It's been my experience with my dry shot that the nitrous works best from about 3500 - 5500 rpms. When you get up around 6000+ you can risk engine damamge from back firing, over revving, etc. My guess is that your back firing is coming from valve float. Maybe you're revving it out too high on the juice? A better ignition would definitely benefit you though. I run a Crane HI-6 ignition with a Crane PS91 coil. The NOS techline said I could keep my stock spark plug gap of .054, even with the nitrous, being that I have an aftermarket ignition. As for what's wrong with your #2 cylinder...your sig states that you have 155K miles on that beast. It's probably getting tired. Especially since you're squeezing it now.