Ok my car was running fine the other day and now it runs smooth until you put it to the floor it pops and backfires through the exhaust. I have a 454 with roller cam and 850 Demon DP Carb. I went through the Hei distributer today and didint fix it, Could it be the demon carb doing it. The engine runs nice a smooth until i open the secondarys then it pops through the exhaust. Any ideas?

Sounds like a lean pop, maybe a vac leak or some dirt in the carb (jet) or insufficiant fuel pressure (need aabout 7psi.) Could also need a valve lash.

If popping through the exhaust,I would lean towards a rich condition.Check your plugs,and if wet with fuel(especially the rear),it could be carburetor or ignition.Check fuel pressure,should be less than 9 psi,and float level.As Gary indicated,also check your valve lash.A lean misfire is usually indicated by a pop in the intake or carb.If popping in the exhaust,it can only be unburnt fuel/air mixture,which is a result of a too rich condition,or weak ignition not completely igniting the mixture in the chamber,and lighting off in the collector or pipes.Hope this helps,Steve