I have an 8 year old Craftsman tractor with 46" deck and 20hp Kohler
engine.

Last summer it started bogging on and off when the cutter was
engaged. This year it can't seem to keep the rpm up at all. I'm not
talking about excessively long or wet grass. It used to cut this
grass just fine.

The engine runs fine when either the cutter is not engaged or it is
engaged and not cutting through any significant amount of grass.

The deck of the mower spins free and smooth (bearing and pullies are
fine) and the blades are sharp.

I've recently replaced the plugs, air filter, oil, oil filter, and a
new fuel filter is on the way.

I'm considering getting a carb rebuild kit but not sure if this will
help condisering the engine runs fine when not cutting.

Any suggestions would be greatly appreciated.