Lawnflite 991 sp6e self propelled lawnmower

SOURCE: SELF PROPEL FAILURE ON YARD MAN MOWER

hello
check the drive cable as not come adrift or broken
you can usually adjust the tension on top of the drive cable
also check drive belt is on the two pulleys
hope this helps

SOURCE: Troy Bilt Self Propelled Lawnmower drive belt

belts usually come off for a few reasons.
They become loose...stretch or the tensioner moves,  Make sure that none of the tightening bolds are slipping.  You can dab them with a bit of paint.
when the pulleys are not in alignment or crooked or bent.  Eyeball the path from one pulley to another, also make sure the tensioner is in alignment and there are no other obstructions.
good, high quality belts are required, and they must be real drive belts...not regular v belts.  If there is a way to watch the belt in operation you can usually see what's going wrong.
On my lawnmower I had to grind some metal away where the belt went from the motor to the transmission.  Good luck 

SOURCE: how do i replace a drive belt on a troy bilt self propelled lawn

You turn the mower over, remove the blade with a 9/16 or 5/8 socket. There is a bolt holding shroud on, 3/8 or 7/16, there are two screws by the rear wheels, 7/16 that need to be removed. Once the shroud is removed, belt can be removed and new one put on.

SOURCE: Self propelled lawn mower inside of drive wheel gears stripped.

Check the drive shaft bearing closest to that wheel.

SOURCE: Craftsman self propelled lawn mower will not propell

Take off the cover up front on the self propel. Engage the self propel and make sure your cable is not broken. If that is ok then remove the wheels and inspect the molded cogs on the inside of the wheels and make sure they are not stripped out. If they are stripped go to www.sears.com and on the tool bar click on parts direct. Under model number enter yours and it will take you to a parts diagram. Get the part number for the wheels and go to local small engine parts dealer and get new. You can also do a search on the web for the wheels and maybe get them cheaper.

Good luck and let me know if this helps or you need any more help.

roundo4