Incorrect Belt Tension
Incorrect belt tension is often one of the main culprits of treadmill problems. When the walking belt is loose, the machine can be sluggish. If you've noticed that the speed slows down when you stand or walk on the moving belt, the belt is most likely loose.
There is a conclusive way to see if your treadmill walking belt is too loose or too tight. On the side of the machine, lift up the walking belt as far as you can from the middle of the deck. It should lift up three or four inches. If it lifts up higher than this, it's too loose. If it doesn't reach three inches, it's too tight.
To tighten your treadmill belt, use an Allen wrench to tighten the two bolts and the very back of the treadmill on either side of the rear roller. Give each bolt a quarter turn to the right. Check how high the side of the belt lifts up. If it needs more tension, keep giving each side a quarter turn until the belt lifts up between three and four inches.
It's important not to overly tighten a treadmill walking belt. Tight belts can cause the motor to burn out, creating more damage and a high-cost problem to fix. To loosen the belt, do the same technique as tightening it, except move the Allen wrench to the left in quarter turns for each bolt.
Crooked Belt Centering
When a treadmill belt is crooked, it can become frayed on the edges and may break. While it's natural for the walking belt to become worn over the years with daily use, a crooked belt will break much sooner.
Fixing a crooked walking belt is done much in the same way that you loosen or tighten the treadmill belt. Rather than use an Allen wrench on both bolts at the rear of the deck assembly, you alter just one bolt.
If your belt is running to the right of center near the rear roller, use your Allen wrench on the left bolt, turning it a quarter turn to the left. Plug in your treadmill and turn it on to a speed of one mile per hour without walking on it. If the walking belt is still crooked, turn everything off and give the Allen wrench quarter turns until it is fixed.
If the belt moves to the left near the rear roller instead, use your Allen wrench on the left bolt and turn it to the right, using the same methods as above.