CRAFTSMAN GARDEN TRACTOR Parts | Model 917276010 | Sears PartsDirect Is your belt part number 137153? I would remove deck & idler pulleys for drive belt, inspect idlers item #50 part # 131494 & item #53 part # 139123 in drive parts list. Bearings are possibly worn out in idlers. Let us know how it goes, thanks.
What mark said plus also check to make sure the transmission support bracket bolts toward the front haven't came out or sheared off. If those come out will allow rocking of the tranny throwing the belt off of the transmission pulley and will also crack the frame where the axles bolt to it.
And other rare things to check
1) engine bolted down tight to mower
2) belt guard in proper place around engine spindle.
And of course correct belt routing.
If a pulley is flat the back of the belt runs on it , if it is V then the inside of the belt runs on it.
Easy to run the clutch pulleys backwards.
Once a belt has run off a few times they get distorted and will jump off with the slightest provocation.
If the pulleys are worn deeply you might need to go down a 1/2" or more in belt size.
The correct size belt assumes you have correct ( new ) size pulleys and correct ( new & strong ) springs.
It also assumes all the pivot holes are nice & round.
all of the above can conspire to make your mowing hell.
