I have a 1970 challenger with a fresh engine build. The engine has 300 miles on it. The engine was dyno'd and broken in by the engine builder. I just changed the spark plugs to NGK v-power. The timing and carb are ajusted and starts/idles good. When driving about 30 MPH and you hit the gas the engine backfires from the rear exhaust. Also if you do a brake torque it will backfire. Any ideas?

The engine has edelbrock aluminum heads, roller rockers, edelbrock performer RPM aluminum intake, quick fuel 850 carb, MSD Distributor with chrysler orange ECU box.

Thanks

 

Check your plug colour, my first thought was power valve.

 

When I changed the plugs I noticed theat plug 1,5,7, 4, and 8 were greyish on color and plug 3, 2, and 6 was damp and dark in color.

 

This is odd, I drew a pattern of the dual-plane manifold and it's like one passage has a wet cylinder and dry cylinder on either end. but it mostly dry on the passenger side of the carb. I,m leaning toward too lean of carb overall but maybe that passenger side has it's own problem. Is the manifold tight to the bottom of carb or with an open spacer. Needs more exploration and more brains than I have.

 

I will check out you ideas. Thanks for the help