Weak/broken valve spring.
Sticky Valves due to fuel being stale.
Valves set at snug .003 int and .005 exh, 1/4" drop past TDC.

Could be blown headgasket into the pushrod cavity also, esp is this engine has 4 head bolts?

Flat cam?

This engine has the 8 head bolts. Valves aren't sticking, are those the specs adjustment wise?

Something you want to remember here, the Compression Release works on the Intake Valve. The INTAKE Valve is bumped or held slightly open late into the compression Stroke to relieve some compression. How much leak back is normal and how much is excessive is something I can't tell from here.

Walt Conner

Yes, good point. I think it's excessive, though, I can hear the air "chuffing" OUT the carb intake, and it'll only run on full choke. Already tried another carb.

Can't say from here if blowing back thru carb is excessive or not but I had one about a month ago that would only run with the choke full on. Turned out it had a pc. missing out of the gasket between the head and intake manifold.

Walt Conner

Yeah, gasket is good, I checked it when I swapped carbs. I'm stumped.