how do adjust the deck drive belt on mtd 38 inch deck?

First of all, you may want to double check the belts part number to make sure you have the right belt.
If you have the right belt then you must do the following. remove the engaging rod and turn the pin head ( that's what it is really called) counter clock wise two or three turns and reinstall it. And start the mower to see if that has fixed your problem. if not the idler pulley spring is installed wrong. you should invest in a manual for your paticular machine so you will be more able to do maintenance on your yard equipment yourself.

Mickey-G TN
July 2008