Outdoor Power Equipment and Small Engines - 6 hp OHH60: bogging down

As many others, im having problems with a 6 hp tecumseh OHH60 engine. i just bought the go kart about a week ago and let the carb sit in some gas along with tightening up the chain and belt. After replacing the carb, the go kart ran fine except for the drive clutch of the torque converter. the clutch seems to not grip unless at about 75% throttle. but when it does catch and the kart starts rolling, it grips alot easier. but thats not the real problem, i just stated that just in case anyone can help me fix that situation also. Well the main problem is that i can drive the go kart on a 3/4 tank and itll run smooth. After about 5 mins, it starts to mummer and die. the more i hit the gas at that point, the more it struggles. if anyone can help me, please respond. thankyou.

Might be one of two things, 1, the gas cap may not be venting properly, not allowing air to replace used up gas, also, 2, after warming up one of the valves not lifting properly due to bad valve push rod clearance.
below is service manual covering your model giving specs on pushrod/lifter clearances.

http://www.cpdonline.com/695244a.pdf
4 Cycle Overhead Valve
hope this helps,
thanks,

Clutch rebuilding is fairly simple on these. Parts are inexpensive. Here is a site with vids that show how to fix the clutch.

http://www.youtube.com/watch?v=bz7nYA-Yo0o