Small Engines (Lawn Mowers, etc.): MTD lawn tractor speed problem, high gears, variable speed pulley

Question
I have an MTD Yard Machine lawn tractor model 13AQ670H088 (TMO-314600), SN 1K196H20176 that won't get up to speed.  In the beginning it would slow down after I had been mowing for a while.  It would finally come to a stop unless I shifted to high gear.   Pushing the clutch in and out would get me restarted for a few moments.  Now it is slow in all but very high gears, and it also smokes the front belt if I keep going at lower speeds.  I have replaced the engine pulley and the rear belt idler tension spring.  I also replaced both drive belts with OEM belts.  I removed, inspected and lubed the variable speed pulley, and also verified that the rear belt idler is spinning free, not running hot, and not catching on something.  I even doubled up the idler tension spring thinking that would surely help, but it didn’t.  I think I have the speed control adjusted correctly, although the owner’s manual that is available on the MTD website for a model 600 is actually for models 660 thru 679 and describes a speed control that is a bit different than mine.  I don’t see any missing parts.  I also dropped the mower deck to reduce the workload, but that didn’t change anything.  This one has me baffled.  Any ideas?  Could the transmission be causing a problem?  Or what else?

Answer
Be sure you are running the engine at wide open throttle.  The next question I would ask is how much grass taking off one tie.  Do you leave it grow tall and then mow short?  If you do you will need to use slower ground although I would suggest raising the height of cut or mowing more often if this is the case.

I assume the engine is operating properly then you have checked the oil and changed it has recommended.  When you say it slows down do you mean the engine or the ground speed if it is the engine slowing down and quitting you may have an engine problem that should be and is the crankcase filling with gas thinning out your oil is oil pumping into your air cleaner from the breather?  Gas in the oil comes from carburetor overflowing and you would need to replace the needle and seat in the bowl carburetor.  If oil is pumping in the air cleaner you have positive crankcase pressure caused by an air leak someplace on that engine.  Check all gasketed surfaces for oil leaks, check upper and lower crankshaft seals.  Check the oil inlet tube and be sure oil seal is intact.  Check the dipstick seal to be sure the or ring is intact.  Remove the crankcase breather be sure it is not full of oil if it is thoroughly clean it and replace it using a new gasket.

If it is ground speed do you have a gar drive or a hydro?