Tractor runs but mower won't run full speed when engaged

I serviced my Simplicity tractor for the season and used it 3 - 4 times but now the mower will not run at full speed when I try to mow. When I try to mow the engine falters. The tractor without the mower engaged drives fine. I am stumped. Anybody have this happen to them and might know what the problem is?

Tractor runs but mower won't run full speed when engaged