Removing tractor deck to change belt and blades?

look for tag on inside of hood for belt numbers. Or get model number off of tag under seat that starts with a 917. then call the 800 number.
instructions... Remove deck, remove pto pulley/ clutch, lock brake, remove crank pully and belt from crankshaft then remove belt from transmission and idler pulleys. Reassemble in reverse order with new belt ensureing the belt is not snagged on any belt guides and all idler pulleys are free.. ( PS sharpen blades while deck is off and check deck belt)

Most common size is 19mm

is the belt turning on the pullies but there not turning or is the belt not turning either
and you may not have the belt idler on right that is what is used to tighten the belt up

http://lawnandgarden.manualsonline.com check this site they may have a manual that could be of some use to you. Good Luck, also the belts themselves probably have model numbers on them that you can use as reference to purchase new ones from either Sears or perhaps a automotive parts store like Lordco

i need a diagram showing for a 20hp yardman tractor model number 13AT605G755

The oil on your pulley could definitely be the culprit. You need friction in order for your belts to work properly.

Download the product manual from the Sears website. Murray manufactured these tractors so you can download the manual from the Murray website also. Procedure for replacing the mower deck belt is quite simple. No need to remove the deck. Spring location also on diagrams. Mower deck parts diagram breaks down the whole assembly. Easier way to remove pulleys is using an impact wrench, otherwise hold the blade while removig the nut that holds the pulley in place. Use a heavy glove to hold the blade for safety!

For removing/replacing the deck belt, the general method is:

1. turn off engine, remove key and set parking brake
2. disengage blade by putting blade engage lever into off (disengaged) position
3. lower the cutting deck to the lowest position (closest to the ground)
4. relieve the belt tension (depending on the model, use a 3/8 inch ratchet set to tighten in the square hole of the idler bracket)
5. remove and replace the belt

NOTE: it's not necessary to completely remove the cutting deck.

Useful diagrams for removing the cutting deck, http://216.213.143.2/ProAuthor/ManProc.cfm?ManualID=425&ProcID=14794

For deck belt diagrams, http://216.213.143.2/ProAuthor/ManProc.cfm?ManualID=425&ProcID=14796

If the transmission drive belt needs to be replaced, several components must be removed and special tools (i.e. air/impact wrench) used in order to change the tractor's transmission drive belt. See an Authorized Toro Service Dealer to have your drive belt replaced.

If you are missing the manual, for the said model,
I have found that the simplest way, of figuring out the correct route for the belt, is to remove the deck, and hold the replacement belt above the mechanism, with the tension from the engine end.
Then 'mock up' the replacing proceedure, bearing in mind the correct direction the tensioner/s, move in.
Then it's not too difficult to figure it out the correct route.
Hope that's clear enough, good luck.

go to the online web site for the yardman this will give you the guide on how to replace the belt.