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2004 Polaris Ranger won't run with air cleaner cover on

I am looking for some help
I have a 2004 500 Ranger, when I brought it home it ran good. When I took it out on the road it started to back fire and miss after about an hour.
I got it back to the shop pulled the air filter out, made sure it was clean, replaced the spark plug and gas filter. started it up and ran ok. about an hour later it started to back fire and miss then it died. would not start. took the air filter cover off and tried to start it and it ran until I went to 1/2 throttle then it started to spit and sputter. pulled the choke out and it cleared up until i took it to 3/4 throttle. I have rebuilt the carb, I have checked the timing and the cam lobs I have replaced the electric fuel pump.
I tried a different known good carb. I even took the drive belt off to make sure the transmission did not have something to do with it. It appears to be starving itself for fuel but what else am I missing? the intake between the carb and head is good (sprayed down with carb cleaner and no difference with RPM's)
After all of the things I have tried I can run up and down the road if I play with the choke and not go above 3/4 throttle and do not have the air filter cover on the air filter housing, Yes the air filter is installed just have to remove the cover for the air filter box.

My 2008 sportsman 500 ho was doing close to the same thing. I cleaned carb even out and didn’t make a difference if I put my hand over the intake tube I could get it to rev up for a second or two. I ended up replacing carb and fuel pump and my ran like the day I bought it.

How's the spark plug look?

I'm confused by the fact that it seems to run too lean but runs better with less air restriction (cover off). Am I correct that this just started, or that it happened the first time you went full throttle? And was the "test" carb identical? I was thinking maybe somebody lowered the valve needle. You never know what a previous owner might do.

After 4 months we still don't know how the plug looks. White, gray, brown, black, other, wet, dry, fouled?