[rustyoldjunk]

I have been fiddling with these foot draggers since the early 90s and some still catch me off guard.
I have a Simplicity 725 (same as B1) doing that to me right now.
I changed and adjusted belt guards,changed idler pulley sizes,adjusted (and adjusted some more) the clutch linkage and belt slack and this one is still fighting me.

 

[rustyoldjunk]

Your belt stop is missing, so no amount of fiddling will solve the issue. You need this horseshoe part, installed to give only minor clearance to the belt when the clutch is engaged (less than 1/8").
View attachment 415233

His belt guard is there Josh and so is mine.
So the fiddling I have done is with all guards in place and still no joy.