Replaced all belts but tractor is still taking off when changing gears instead of sitting still until gears change

Is the foot pedal adjusted correctly to where when you press it down to change gears the pulley gets pulled away far enough?.

May just need adjusting..

Check it when the engine if off and press the clutch or have someone helping you press the clutch down and see how far the pulley moves.. good luck

If your MTD is anything like my Troy-Bilt (also made by MTD), this might help:

Mine has a rod connecting the drive pedal to the idler pulleys. When you press on the drive pedal, the rod pulls on the pulleys to put tension on the belt. When you release the pedal, the rod pushes the pulleys to release the tension. At the end of that rod is a ball joint that screws onto the rod. If the rod is too short (screwed too far into the ball joint), the pulleys don't get pushed far enough and there will still be a bit of tension on the idler pulleys and the belt. As soon as you put it in gear, the tractor wants to move even before you press the drive pedal. I had that problem until tonight. I unscrewed the rod from the ball joint leaving about 3/8" in the ball joint. That made the rod about 1/2" longer and that was enough to let the drive belt go slack until I press the drive pedal.

I hope that helps.