Out of Ideas

Hello

I have a Duromax XP10000e generator powered by a 439cc engine.

This generator has about 5-10 hours of use on it. Not too long ago when starting, it started surging and after a few minutes it would die. It was a an absolute pain to restart only for it to die.

I tore down and cleaned the carb. No difference. I've since replaced it with two more brand new carbs and gaskets... no change. I replaced the gas. Nothing. I even replaced the fuel line, no change. I've checked valve lash clearances to make sure they're in spec. I've inspected and checked the gap for the spark plug. No change. A compression test showed 90psi. I confirmed thatbthe governor isn't responsible by disconnecting it from the carb.

I can start the engine cold realatively easily with the choke and throttle fully closed. It will only run after that with the choke fully off and keeping the throttle closed. Opening the throttle causes the engine to stumble and die with a second or two. I've noticed that with the throttle open, fuel spits out. I can keep the throttle open and the engine running with staring fluid, though it backfires thru the carb frequently. Finally, with the engine off I can feel air pushing out thru the carb when pulling the starting cord. That last detail seems to be the culprit, but what would cause that because I confirmed valve clearances and compression?

Much thanks