1980s Simplicity 6216 Garden Tractor loses drive power


My Simplicity 6216 garden tractor doesn't get up to full speed and actually loses speed on an uphill grade. I have adjusted the clutch and brake according to the manual.... http://www.simplicitytechpubs.com/6767PRINT/PDF_files/TP_100_3001_00_XX_S.pdf ...and have not made any progress.
I have cleaned the variable pulley and replaced the belts. Transmission fluid is up to the proper level.
When mowing or plowing snow, the ground speed increases very slowly but does not top out.
When the clutch/brake is released, the tractor starts to move. As expected, speed increases. As it does, the pedal will slowly raise and when the tractor is moving full speed the pedal should be all the way up. However, the pedal never comes all the way up and full speed is never achieved.
When climbing a hill in the yard the ground speed decreases amd on level ground it increases somewhat. On a downhill grade the speed increases somewhat.
I think the symptoms get worse as the engine warms up.
Any ideas what the problem is?
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Just a wag, but it sounds like it's loosing power under a load, which is what the governor compensates for. See if you can find info on adjusting the governor

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