I'm pretty sure it is a bad idea to run a stock coil with a MSD driving it but how long do they last and what are the failure symptoms?

Reason I ask is that I changed heads on my racecar and when I put the engine/tranny back in and got everything buttoned up - it ran like crap. Didn't like to rev and would miss and backfire a lot. The backfire wasn't coming up thru the carb so much as hearing it in the exhaust.

Am pretty sure it is ign system related but dizzy wasn't touched and I confirmed all timing and firing order.

When idling we pulled wires off the dizzy one by one. When removing number 5 the sound of the engine didn't change at all - when connecting it back up you could see the spark jumping and it just looked weak - like it wasn't enough to fire the mixture good enough. Also the popping in the exhaust sounds just like unburnt fuel igniting in the pipes.

I just installed a MSD Blaster 2 and put a couple new plugs in 5 & 7 and so far everything sounds good. I am awaiting my call for time trials so will feed back how it performs going down the track soon...