The John Deere GX85 and SX85 PIN riding lawn mowers are each equipped with a primary drive belt that powers them forward and a mower deck drive belt that turns their cutting blades. If the mower deck drive belt slips on its pulleys, replace it with a new mower deck drive belt because it cannot be adjusted. In some cases, a loose primary drive belt can be tightened by adjusting the mower's idler arm.
Put Safety First
Remaining safe while working on your John Deere riding lawn mower requires following all safety warnings in its owner's manual and taking a few safety precautions. Prevent the mower from rolling forward or backward by parking it on a level surface and engaging its parking brake. Working on the mower only when its engine is turned off and the ignition key is removed from the ignition will help prevent injury from moving parts while you adjust the primary drive belt.
Access the Primary Drive Belt
A primary drive belt that has 1 1/2 inches or more of looseness, or free play, can be adjusted. The John Deere company does not recommend adjusting a primary drive belt when its free play is under 1 1/2 inches. The amount of free play in the accelerator pedal, which is on the fender, corresponds to the amount of free play in the primary drive belt. So you can determine that belt's amount of free play by measuring how much "give" is in the accelerator pedal. Begin that measurement by placing the shift lever in the "Forward" position with the engine turned off. Push downward on the accelerator pedal with your hand, using moderate pressure until the pedal becomes hard to push. Hold the accelerator pedal in place at that point by using one hand, or have a helper hold it, and measure the distance from the side of the accelerator pedal that faces the front of the mower to the front of the slot in which the accelerator pedal is located. If the distance is 1 1/2 inches or more, then the primary drive belt needs to be tightened.
Tighten the Belt
A primary drive belt stretches over time from wear, causing it to slip and resulting in slower ground speed for the riding lawn mower. Restore your riding lawn mower's ground speed by adjusting the idler arm. The idler arm looks like a flat bracket and is attached to a large pulley in front of and near the outer edge of the rear wheel, on the riding mower's underside. Access it by lowering the mower deck to its lowest position. Adjust the idler arm by removing its bolt that is nearest the outer edge of the pulley. Removing the bolt allows the idler arm to slide. Loosen -- but don't remove -- the bolt immediately next to the bolt you removed; that second bolt is toward the center of the pulley. Push the idler arm toward the center of the mower, and put the bolt you removed into the alternate hole on the other side of the bolt you loosened. Tighten both bolts.
Test-Drive the Mower
After you adjust the riding lawn mower's primary drive belt but before you the mow grass with the machine, it's important to test the mower to ensure you can stop it properly so you avoid injury to yourself or your property. Put the mower's transmission lever in the "Neutral" position, and push the mower to an open area of your lawn where no chance of running into trees or other objects exists. Sit on the driver's seat, start the engine and place the shift lever in the "Forward" position. Press the accelerator pedal with your foot to move the mower forward, and then take your foot off the accelerator pedal. If the mower stops moving forward, then the primary drive belt was adjusted properly. If the mower does not stop moving forward, then return the idler arm bolt to its original position because the primary drive belt is too worn to adjust and needs to be replaced with a new one.