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Hi all Been trying to figure out why the drive belt comes off the transmission pulley when I start the mower. The mower is a 2009 Troy Bilt self propelled rear wheel drive 4 speed model 839 series 12A1839F011.The drive cable doesn't fully return to position, I can move the lever its attached to back so as to take the slack out of cable. It is also possible that the speed selector was moved before the engine was running. Any thoughts or ideas as to the problem would be great. Thank you

Lever not returning probably means the idle wheel tensioner is not full engaged leaving the belt a little loose.

Make sure cable or arms whatever the lever attaches to are lubed and moving freely.

Take a look at the idler pulley, if you can see it, and have someone operate the lever to check for movement and belt tension.

ARIPartFinder Click anyway, remove deck & then remove idler pulleys for drive belt & inspect for replacement, bearings in idlers wear out, use OEM Troy Bilt parts. Let us know how it goes.

There are NO idler pulleys on this machine!!! There are 2 pulleys only 1 split pulley on the bottom of engine and 1 split pulley on top of transmission. The belt comes off not when the engine is running BUT when the cord is pulled to start the engine. It seems to me that the transmission is stuck in gear if that's possible.

ARIPartFinder For safety remove spark plug boot & turn mower on its side with carburetor side of engine at top, inspect underside of mower parts, look for loose, missing, or broken parts. If underside is ok turn mower right side up & place a board or two under mower deck so rear wheels are off ground, remove wheels etc. & inspect all parts, take your time doing this be patient & organized keeping right & left side parts in order, you may have to remove transmission for inspection & see if axle will turn as well as pulley. Let us know how it goes.