Sounds like fuel starvation — is the carb bowl full at all times ?

Sometimes older coils have broken wires inside — when they heat up they misbehave … do you still have a hot spark after the motor stops ?

I’ve never been up close and personal with a K-80 — if it has an exhaust cutout are you running with it open or closed ? Closed creates more back pressure.

My K-70 and A-65 suffer from carbon buildup where the engine block joins the leg — it creates a lot of backpressure and therefore exhaust gas hangs around along longer in the combustion chambers.

WRT to the 60 lbs compression — it probably won’t run well with either the cutout closed or clogged exhaust ports.