This b&s mtd riging mower runs good but, it won't go up hills. When on high it won't mow it just has no power. What can I try to do to help it out without rebuilding.

This b&s mtd riging mower runs good but, it won't go up hills. When on high it won't mow it just has no power. what can i try to do to help it out without rebuilding.
Typical reasons: Blocked air filter, choke not fully off

Is the problem the engine, or the transaxle? Do you notice any jerking, or sounds out of the ordinary while operating?