I have an MTD Yardman lawn tractor 400 series that the rear drive belt is coming off.I put a new belt on same problem.It appears to me the problem is the idler is quite a bit higher than the transmission pulley,in fact I know that's why it's coming off you can watch it.But there is no up/down adjustment for either pulley and I am hard pressed to figure out what could have cause this misalignment.I can align them by lowering the idler by shimming the mounting bracket downward or shim the transmiision pulley upward but am stumped as to what caused this.

Hi,
The transaxle is held in and stable by two bolts at each axle housing abd two bolts holding the front center to the crossbrace. It is those last two that are missing and allowing the torque to twist the pulleys out of alignment.
Peace,
Clarke

I have repaired 2 different MTD mowers with this problem. I probably went overboard on my solution, but I guarantee that it works.
The transaxle bolts are bending the thin frame that they're mounted on, which causes the tranny to flex, thus throwing the belt off the top. I added a 1"x 1" x 12" angle iron to the top of the frame and a 1" x 1/4" x 11" flat steel to the bottom of the transaxle, that bends up to meet the underside of the frame and angle.

1. Put the mower on jackstands and remove the back wheels (Good luck with that). You may need a wheel puller, a sledge hammer and a torch!
2. Before removing the bolts from the trany, bungy cord the axles together over the fender. Put the wheelbolts back into the axle to hook them to.
3. Lay the angle to be even with the back of the frame, mark where the holes need to be drilled to match the axle mounts. Discard the small plate on top (I used them for drill guides). They're not centered to the angle! The holes will be close to the inside of the angle, just enough to get the nut on! "hoggering" may be necessary.
4. Drill matching holes in the flat bar and one hole on the opposite end. This is for the front bolt and it doubles as a pilot to drill through the frame and the front angle.
5. You will need to bend the flat bar in a "Z" like pattern to about a 2 1/4" parrallel that will lay flat to both frame and bottom of tranny.
6. Reuse the 5/16" tranny mount bolts from the bottom up. Use Loctite on the bolts, lockwashers may not fit. Mount the front with 5/16" x 1" bolt.

I cleaned and painted all the parts before assembly. If you would like to see pictures of before and after, email me and I'll be glad to share them.

Hi,
Yes I dare say you went overboard since you do see the frame fracturing but have not found the true cause. Depending on the age of the machine, there are one or two bolts/screws that attach the trans to the cross member that is at the front of the trans. These keep it from flexing. Older models use a 1/4" X 1 3/4" Gr5 fine thread bolt, washer, lockwasher and nut(I use a Nyloc nut personally.) Newer models use two 5/16" X 3/4" self-tapping bolts.
Peace,
Clarke