The car Idles good (1100rpm) and drives good as long as you stay out of the secondaries. It will backfire through the carb like crazy if you try to feed it the secondaries. The backfiring happens whether you are driving it or reving it in park.
The engine is a 454 BB chev 10.5-1 cr, 294 comp solid cam, 850 speed demon carb, MSD 6A ignition, 1981 Camaro. This is my cousins car and he is pulling his hair out trying to fix the problem. Any help would be great.

Thanks
Dino

I would pull the valve covers off while it's running and watch the rocker arms and make sure they are all moving the same way. Could be a cam problem, if you see 1 or 2 rockers not moving much thats probably it. I am assuming you have checked all the obvious stuff, the timing and firing order.

It could be lean in the secondaries.

Thanks for the replys, we thought about the lean on the secondaries thing but we jetted all the way from 86 to 99 jets on the secondaries and it doesn't change to much, if any at all. On the highway you can slowly feed it with the primaries to about 5500 or so, but as soon as you nail it pops through the carb. Would a broken valve spring act like this. We have checked all the wires, ohmed them out and made sure they are in correct order, replaced the intake gasket.

any ideas

Thanks Dino