What's Causing My Backfire?

[PeasAndCarrots]

1986 Chevy G30, 200k miles. After a recent 200-mile highway drive, engine backfires and puts black smoke out the exhaust. Suspected head gasket failure between #5 and #7 cylinder, but replaced HG and problem persists.

Low compression in #7 cylinder, half-burned fuel shot back up #7 intake runner. Valves are fully seated; springs, pushrods and rockers passed visual inspection.

Ideas? Thanks in advance,
-joe & christy

[john8662]

Highly suspect valve issue here with the low compression and the popping in the intake runner.

Inspect the entire tract of that cylinder's intake intake and exhaust valve:

Valve, spring and damper spring, pushrod (straight), lifter down below, remove and inspect. I'm not sure that collapsed lifter could cause a backfire, possible.

[350Tacozilla]

internal crack in between intake runner and exhuast?? did you have heads pressure checked when doing the HG's??