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?

make sure all idlers and spindles on your deck are turning free, possible blockage in carburetor, the engine demands more fuel when the blades are engaged.