What causes backfiring?

[tommyboy]

Ok , my question has to do with actual side pipe temperatures. I just took my maiden voyage with my SPF 1593 car. Engine is a Dart 351W block bored/stroked to 427cu. in., 48IDA Weber carb induction, complete roller valvetrain and AFR 205 heads. Total timing has been set to 38 degrees advanced, all in by about 2700 rpm. I was extremely careful at initial startup. I set up all the carbs air flows and idle mixtures. Double checked the timing and also pointed fans at each side pipe so as to make sure there was no extra heat to bubble the paint. Anyway the car started up real easy and idled perfect. After 20-30 minutes of run time and no leaks, I shut it down and changed the oil. I then started it up checked everything agian and went for a 12 mile test (shakedown) ride. All seemed well until I got upto around 45-50 MPH and about 2500-3000rpm and started hearing some pretty noticable backfires through the exhaust. So I headed home shut it down and was going to check my header to sidepipe bolts and recheck my timing. The one thing I did notice was that my side pipes seemed very hot where they exit the fenders. Hot enough where I did get a bubble in the paint down low, somewhat out of sight. Needless to say the ride then turned bittersweet. I put my fans back on the sidepipes and that dissipated the heat pretty fast. The overall engine coolant and oil temps were very good. The carbs are setup more on the rich side. Timing and header bolts are ok. So how does one keep those pipes from bubbling the paint? The clearance around the pipes, where they exit the fenders, is good.