just rebuilt runs bad

Are they new plug wires? an old or damaged set could leak spark to another cyl. just throwing it out there..

AND as MarksMopar and others have stated,, a distributor installed 180 degrees out... WILL NOT RUN,, sure will backfire though.... lol

I think 70aarcuda's on the right track,, that noise being the vital clue,, possibly valve sticking open,, then snapping shut,. the backfire coming from the spark, and the valve being snapped shut by piston contact.

If you can run it with the valve covers off,, put your finger on the side of the valve retainer or on the rocker tip, of each valve,, trying to sense one that feels different.. this is how I find the loose valves at adjustment,, you can feel the loose one by the tap. you'll be able to feel it snapping shut. hope it helps, if not now, in the future... cheers
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