MCI motor wont run with air filter cover

[brokebuggy]

I have a cart with the MCI motor in it. came in with a no start, found bad wire on the starter and got it running. cart would start and rev up in neutral but in gear it would barely move and the motor will not build any RPM'S. I have cleaned the carb, drained and cleaned the tank, adjusted valves, replace air filter and no difference. Ran with a spark checker on both cylinders and they are both firing. If I remove the air filter cap an d leave the air filter in the cart runs as it should , put the cap on it and it will run just enough to make it move. And yes I also checked the air intake hole in the cap. It is as open and as free as can be.....

[brokebuggy]

Well tried a new carb last night and still no difference. Why will this not run with the air filter lid? I am at a total loss.......

[meimk]

Have you checked compression? Worn rings, scored cylinder walls, hole in a piston, broken connecting rod. Those will run, but have no power, on just one cylinder.

[brokebuggy]

Well crap,This is what I get for being close minded. Figured since it ran good without the air filter, really started right up had decent power and speed. So I never checked the compression ...75 lbs both cylinders