The John Deere 185, long out of production was one of the square bodied lawn tractors equipped with a single blade mower deck. Given the age of the mower, it is likely the belt requires replacement. If you hear prolonged high-pitch squeals when you engage the mower deck's blades, you know the belt has worn beyond adjustment. The easiest thing to do to remedy this situation is to replace the belt with a new one.
Locate the mower blade lever and turn it to the "off" position to stop the mower blade and to initiate slack into the drive belt.
Lift the deck lever upward to raise the deck and then place wooden blocks under the deck. Lower the deck lever to place the deck onto the blocks.
Remove the butterfly pins from the linkage rod pins and then pull the linkage rods off the linkage rod pins. Lower the linkage rods out of the way.
Position yourself in front of the John Deere so you can see under the front of the tractor. Locate the drive pulley. Pull the drive belt off the pulley. To do so, manually rotate the pulley while you pull the belt off.
Slide the deck out from under the tractor and then take the old belt off the deck. If you are not sure how to route the new belt, make a diagram of the belt before you take it off. It is likely; the diagram printed on the deck wore off due to age.
Slide the deck back under the mower and attach the drive belt to the front pulley.
Reattach and secure the linage rods to the linkage rod pins. Use the butterfly pins to lock the linkage rods in place.