Peterbuilt

After installing a chambered exhaust on my 74 (automatic with no AIR) I had the same afterfire noise.
I solve the problem by following Lars suggestion and adjusting the mixture screws.
I did not have to adjust the APT.

See post 23:
https://www.corvetteforum.com/forums...exhaust-2.html

MBrianB

Finally had time to check this out.

The orofice directly behind the fuel filter inlet, just above the idle screw on the passenger side is ported. i.e. no vacuum until the throttle is opened.