Hello there.

I've got a Cub Cadet SC621E (21 Inch, 6-Speed Rear-Drive, Electric Start) walk behind mower. It's older, from 1999, and has the Briggs engine.

After changing the drive belt, replacing both clutch and gear selection cables.

The self propelled drive works fine most of the time. When I'm in the middle of mowing the yard, it will usually act up and I'll have to repeatedly engage and release (a few times) the drive handle for it to take back off. It will just sit there, yet I can hear the engine bog down a bit from the engagement of the clutch... It usually helps if I select 6th gear, and then engage the drive handle when this problem happens...

My main problem (because I have a yard with quite a few obstructions) is the fact that the wheels tend to lock up when pulling the mower in reverse (with the drive handle disengaged, of course). I'll be mowing fine, but when I want to pull the mower backwards, the mower will usually go about a foot, and then the wheels lock up. Sometimes I'll be able to go 10 feet, and then it'll lock up. Other times, as soon as I start to pull backwards.

When the wheels lock up in reverse is usually when I have a problem going forwards again. After they lock up and I press the drive handle again, I'll usually hear a loud click from the mower and then if I press the drive handle again, it'll take off.

I've taken apart the gearbox, cleaned the chain and sprocket, re-greased them, as well as cleaning the other gearbox with the pinion gear, cleaning out the old grease and putting new in.

I've heard that there's some sort of pin or something that slides out (or in) when the drive clutch is disengaged to allow free movement of the wheels, and when the drive clutch is engaged, the pin locks back to allow the transmission to turn the wheels.

Any ideas?