I have just fitted a new transmission belt to my westwood 1012, it will go into gear when the motor is off but when the motor is running the gears will not engage when the clutch is pressed

SOURCE: Clutch issues: release clutch and mower doesnt move.

"....I just took off the right rear tire to replace."

Make sure the key hasnt fallen out of the right rear tire. It has to have the key to work. Its a peice of metal about 3 inches long and flat on all sides. It should slip into the grove on the tires shaft. Hope that helps.

SOURCE: gear of starter motor won't engage in flywheel to run the tractor

Look under 'My Profile' (Longhorn294 in the top right corner of this page)on page 5 at this post and solution. "craftsman riding mower starter will spin but not engage". Dwain

SOURCE: Craftsman riding mower won't move.

yes it is a bad belt unless it is a hydro-stat(no clutch) you need to look under the mower and see if the belt is on correctly

SOURCE: starter gear doesn't move up to engage flywheel gear when started

The solenoid does two things -- it closes an electrical contact to run the starter motor, and also pulls a lever to slide the motor gear up to engage the flywheel. Since the contact is closing, the solenoid itself must be OK, but the lever could be broken, or the sliding gear could be binding or stripped. Since you're looking at a probable replacement, try cleaning with compressed air, then squirting some WD-40 into the starter gear area - you could get lucky! Otherwise, you can replace the starter/solenoid assembly -- you can find replacements on eBay for about $80. Less expensive but more time consuming is to remove the assembly, disassemble, and replace just the defective parts if you can find them. Good luck! Please rate my answer if helpful, thanks.

SOURCE: stalls when put in gear or engage mower

had same problem is safety switch by the deck causing your problem jump it or replace it