First check that the stabilizing arm is attached to the transmission
the arm is there to prevent the transmission canting forward & throwing the belt
I doubt it is the problem , but a check costs nothing .
Second check that the spacer has been put under the pulley
As the belt jumps off the top that suggests the pulley is too low
If it is there pop a couple of washers in under the pulley to see if that fixes the problem
Lastly as the pulley is fixed you can make a belt guard from a piece of flat strap with a right angle bend
It con come from the top but needs to be similar to the one on the moving pulley
It should be about 1/2 way between where the belt enters the pulley & where it leaves the pulley.
Nearly every other mower would have one there but none is shown on the OPL so it looks like MTD is just being cheap .
