1998 Arctic Cat 300 won't run right after warmed up

[GlennMcl]

1998 Arctic Cat 300 4x4 Red

This quad formerly ran fine but started to run poorly as it wouldn't idle or rev easily.  The current problem is as specified below.

Starts and idles fine when cold.  Will rev to mid range when cold.  As it warms up it will not rev and idles poorly as it gets warmer.  I thought might be a problem with the ignition coil so I replaced it with a new one, no improvement.  It doesn't backfire or stutter.  It just falls on its face when the throttle opened.  Also no smoke out exhaust.

I changed the carb to a new Mikuni VM32 which is a common swap. Fuel tank was cleaned, a new shutoff valve, new fuel filter, new fuel line installed.  Air filter, ignition coil are new.  CDI was recently replaced with an aftermarket unit.

I verified fuel is flowing from tank to carb.  Fuel tank vent is not clogged.

Still no improvement.

Possible valve adjustment problem?

Any advice would be appreciated.

Edited June 14, 2021 by GlennMcl

[GlennMcl]

Just discovered the engine crankcase is full of gas. Young one never told me the fuel tank shut off valve was stuck in the open position.  Tank was empty when I changed the shutoff valve and fuel line.

[GlennMcl]

Drained gas from crankcase and gearbox. Cleaned oil pump screen, blew out oil cooler lines, changed oil filter.  Runs great now.  It didn't like 2 gallons of gas added to oil I guess.