Misfire at cruise, popping/afterfire at 3000RPM

AJ/FormS said
what heads are on it?

Just stock cast iron 440 heads. I didn't build the engine, and unfortunately the guy I bought the car from didn't have a build sheet. No cam card, no nothing.

67Dart273 said
If you did not break that plug removing it, that is suspect. The two wires NOT to bundle for a certainty are 5 and 7

No sir, I saw that it was broken before I removed it. HOW it broke, I have no idea. The wire set I bought came with ceramic boots, and the #6 boot is also fractured.

I'm afraid all I had time for today was the compression and leakdown tests, so here are the numbers from the leakdown:

1 50%
2 40
3 40
4 85%
5 35
6 35
7 35
8 45

Clearly there is an issue with #4. I couldn't tell if it was leaking through the intake or exhaust valve, but certainly wasn't coming from the oil dipstick tube or radiator. I connected the hose directly to 100% shop air, and I could hear a loud rush of air in the cylinder head. Nothing coming from the header or carb. Not that I could hear at least.

I'll study everyone's notes tonight and get back after it tomorrow.