Craftsman 6 speed can't change speeds.

[GeeMSee]

I have a 1999 Craftsman lawn tractor with 20HP B&S, 42" deck, 6 speed, 917.270930. I can only change speeds with the engine off - not running. I was bagging grass, cutting in speed 3. Stopped to empty the bags, left the trans in 3 instead of neutral, set the parking brake, emptied the bags. It then started chattering, jumping forward, and a bit of smoke, since then I can't change speeds with the engine running. Speed lever moves fine, all 6 speeds, neutral and reverse with the engine off. Any ideas for what to look for? I have to keep this one running for a couple more weeks 'til the new tractor arrives.
Thanks

 

[BigMike50]

for what ever reason, when you disengage the clutch, the belt is still tight on the trans pulley. That is causing tension not allowing you to change gears while motor is running. With the engine off, have someone engage the clutch fully and check to make sure there is a good 1inch of slack on the trans pulley. This is exactly what happens when you put too tight of belt on the tractor. not saying this is your case but something is causing tension and you also said you saw smoke

 

[GeeMSee]

Thanks for the quick reply. I was thinking along those lines.
After I dumped the bags the mower started to jump/creep ahead I released the parking brake,
It was hard to put into gear and the belt was smoking a bit. Belt is a couple of years old, I'll
Throw the old one on tomorrow.