Originally Posted by DaleR
I have a ’72 402 BB engine. The engine was totally rebuilt 5 years ago and I installed new rod bearings, rings, mild performance cam with timing gears, lifters, and ground the valves and seats. When the engine was fired up it ran like a fresh motor, not issues. It has a stock points ignition and a 4 barrel Qjet.
The engine then sat for 3 years due to unexpected issues in life. I recently rebuilt the carb to insure no stale gas/varnish issues and fired it up. It runs but with consistent muffler explosions. I insured that the distributor was installed correctly ( #1 at TDC with rotor pointing to #1 tower). Timed it to 12 degrees BTDC , no change, still exploding. Rotating distributor full travel, no change. Vacuum gauge at 15 in (in late timing indicator). Ran a compression test, 110 psi across the board. Anyone have a clue as to what the issue could be?
Double check the firing order to be positive the plug wires are correct at both the cap and at the plugs. Number 5 and 7 are next to each other and are often swapped accidentally (see drawing below). Even if you think they're right I'd want you to be absolutely sure before going any deeper.
Be sure the cap is spotless inside and not cracked. No cross firing plug wires allowed either.
Removing one plug wire at a time while the engine is running will tell you if this is isolated to a particular cylinder as has been said already.
FWIW, the compression test showing 110 psi may
be low, but this also depends on the cam timing. A 'lumpy' cam will tend to pump lower than a mild/RV cam.