81 KZ750LTD H2 - Cylinder 4 is backfiring into the Carb

Hey everyone, got a weird one. I’ve gotta 81 KZ750LTD H2 and cylinder 4 is backfiring into the carb. But, only when the engine is warming up and the RPMS are falling. If you rev and hold, no cough, only when falling. And after the bike warms up for 10 minutes it stops entirely. You think it’s the intake valve?

Lean, clogged pilot or tight valve.
Steve

Maybe sucking false air too.

I think he still has his winter air filter in. Might be his blinker fluid is low.