Small Engines (Lawn Mowers, etc.): Murray riding lawn mower, transmission problems, transaxle

Question
I was given a 3 year old Murray model 40373x51b.  The guy said it had been in the shop for transmission problems, was fixed then went bad again.  I got it home and started it and no movement.  I tried to move up the blade housing and when I started it again it moved slowly forward and back.  Is there something I could adjust under the deck that would help it move?  Is there a drive belt that may be loose?  And, if so, how do I adjust it?
Thanks for any help you can give.

Answer
Parts diagram:

http://www.motoruf.de/mo/ersatzteillisten/pdf.php?file=Murray/./40373X51B.PDF

I see at least one spring that could cause the belt too be too loose.

What is the transaxle model number?  The link below will help you locate model mumber.

http://www.smallcarplans.com/manual.pdf

Let me know.
Eric