Backfiring help

[krcaddis]

Yesterday my '85 C-20 Scottsdale blew the muffler open along the seam on deceleration after a series of lesser pops. It's very first few miles on my 1st gen Mexican crate 350 transplant - driving to town to smog it. Then the plastic shield on the tank caught fire, then the cap. Fortunately a rancher was there, then the FD and damage was not the catastrophe it might have been. Didn't make my smog appointment.

I figure timing or carb was the most likely cause of the backfiring. It ran very well, idles perfect smoothly with lots of power. Carb is a freshly rebuilt Holley, professionally done and set up on a test mule. Changed the balancer to have belts aligned, and re-marked TDC. Timing was adjusted for best idle sound. Seemed to backfire only when engine warmed up - maybe only 3-4 miles. Exhaust manifolds don't leak, I'll check valve operation, plug wires, dist. cap.

Any other suggestions?

And now I need some parts:
heat shield for driver's side tank,
black rubber trim strip across the fuel door (only).

Anyone know of a good source? These aren't the usual restorer buys, and LMC doesn't show them. Used is fine.

 

[1987 GMC Jimmy]

I’m sure someone on here could set you up with the plastic piece or tel you where to find one. A lot of folks throw them away because they think they’re a nuisance. Does your truck have the trim painted black? It was silver and smooth through ‘88, and then they switched to a ridged black trim for the last few years of trying to be like the GMT400s. Everything that you’re going to check should be checked so you’re on the right track there. I’d check the timing with a light. By ear can get you close, but the light does it right. I’ve heard of timing chain issues making them pop through the carb, but it maybe worth considering. I had this issue on a 454 in an RV (minus the engine trying to turn me into a roasted skeleton), and it was most definitely the carb. That leads me to suspect an issue with the tuning.

 

[highdesertrange]

unburnt fuel being dumped into the exhaust plus an exhaust leak or faulty smog pump(O2 allowed into the exhaust) = boom(explosion) in the exhaust system. highdesertranger