The belt only has to have enough tension to avoid slippage. Excessively tight belts introduce drag and will slow down the deck. Any of the 3 belts can affect the speed. I wouldn't worry about adjusting the flywheel end float - it shouldn't need adjusting.
yes jimmyjimmy19702010 the belts are actually fine if anything to loose old age, the capstan tube that has been replaced was to long, I replaced the tube and belts but still exactly the same, looking at the deck I'm sure its got the wrong motor, the other 9191 I'm working on the motors smaller and has speed adjustment in the back of the motor the one iv got problems with doesn't,,, does anyone agree its possible the motors wrong, also appears to have a plastic arm missing that determines FF & RW god its a pain.
unsure but everytime I try uploading images it places them the wrong way around does my head in......,,,😕
PS the motor that has issues is the large one, without speed adjustment in the motor,,,,,