Outdoor Power Equipment and Small Engines - Craftsman snowthrower throws drive belt when in reverse

I am getting reading ready for winter early. I have tuned up my snowthrower, Changed oil, spark plug, and belts. It is a Craftsman 6 horsepower two stage snowthrower, model # 536.888400. When I put it in reverse and engage the drive it will back-up a few feet then it will throw the belt off. I am sure that the belt is properly routed on the pulleys. I have disassembled the upper pulley and moved a washer into it to widen the v groove. Then I tried it again and it still throws the belt off in reverse. I bought the new belts from sears, the numbers match the old ones and I am sure that I am installing them correctly. Does anybody have any idea what could cause this. Any help would be appreciated.

You better make sure you have the pulley and washer and spacer arrangement correct. The order is (beginning at the engine): washer, pulley half, pulley half, washer, washer, spacer, pulley half, pulley half, flat washer, lock washer, bolt. There is not to be any washers in between the pulley halves. FYI, the wheel drive belt part number is 579932 and the auger drive belt part number is 581264. How is the tension idler pulley and spring and arm? Pulley bearing good and free? Idler arm pivot point free? Spring intact and good tension?

The reason I moved the washer into the middle of the pulley halfs was because the new belt was riding partially out of the groove. The part # was 579932MA for the drive belt and 581264MA for the auger belt. The tensioner spring seems strong enough. The bearing is free wheeling. The arm looks good and pivots quite good.

With belt tension relaeased (from the wheel drive train) take a hold of the driven pulley (lower) and check for side play (up and down movement) of this pulley. If excessive then you have shaft bearing issues that needs to be taken care of. Also, be sure the two engine belt guides are at about 3/16" from the belt (when the belt is tight). You will not likely find belt guides at the driven pulley but may want to consider installing two such. Is the belt falling off the drive (engine) pulley or the driven (lower) pulley?

I removed the belt and the lower pulley has very little play, I'd say about 1/8 of an inch. The auger engine pulley is much larger diameter than the drive engine pulley, so the belt guides cannot get any closer than 1/2 inch to the drive pulley. I don't have anyone to run the snowthrower for me so I cannot see what pulley it jumps off of first. I am going to put the old belt back on and see what happens then. I compared it to my neighbors snowthrower and is the same setup. but his doesn't have this problem. Thank you for your help so far. I'll tell you what happened in about an hour.

I put the old belt back on and made sure the pulley was clean and rust free. It works just fine with the old belt. But what happens when the old belt finally brakes or wears out. So I put the new belt back on and it pops off when driving. I did this about 10 to 15 times and it finally stayed on for at least the last ten times in a row. I hope this won't happen when I use it this winter. I will save the old belt. I believe the new belt just needed some break in time. Anyhow the morale of the story is if it ain't broke don't fix it!!!!! Thank you for all of your help, puey61!!!!

Something still doesn't seem right Wiz and now is the time to rectify it. I know I'd be cursing come first snowfall and the thing is useful as *%#@ on a boar. With the engine running do you see any wobble in the driven pulley? How is the alignment of all three pulleys? If the driven and idler pulleys appear aligned but not the drive pulley with those then you can move washers around to help with this. This holds true for the auger belt pulleys alignment as well.