briggs only runs for a short time

the mower is a rally 20'' with a 9s502 briggs and stratton 3.5 horse engine and the pulsa-prime carb only ran off the prime and i rebuilt it and it still did the same thing so i put an old carb off of an older mower and it ran great for a short time but when halfway through cutting the grass smoke came out of the pcv and it would only run on half throttle and the engine would bog down then it would only run if i choked it all the way and pulled it alot of times when i took the oil out it came out brown grey and chunky. so whats the problem how do i fix it

i changed the oil and cleaned the carb and now it only runs if you hold the throttle plate shut with a screwdriver

One might look for oil leaks around the cylinder head, it could well be a head gasket leak.

shoots liquid gas out of the exhaust

ive been making an attempt to start it every day and it wont ive done a wet compression test and it was 40 psi and i took the breather off and the valve springs are fine and i noticed oil and dirt around the cylinder head and then i remembered seeing something about a cylinder head gasket on here and i took the head off and the cylinder head gasket was destroyed so i just ordered a new one for like $3 new but another problem is the engine floods after like a few pulls and does not fire i checked and it has spark but it shoots liquid gas out of the exhaust what causes that and how do i fix it