Next jump the belt at the sprocket pulley at the opener 2 notches on the belt.
Too much slack in garage door belt.
If you have an 8ft door you should have belt number 41a5250 1.
There should be no slack in the belt unless it s too large.
The chain or belt should pick up slack.
This would reduce the slack and less tension on the belt.
The specific procedure should be in the manual for your unit.
Tighten until the chain or belt is about 1 4 inch from the bottom of the rail.
If you have a 10ft door you should have belt part number 1a5250 2.
The manual does not indicate how to solve this problem.
If you find that your garage door opener chain has become loose with time you may want to tighten it up in order to remove some of that slack.
The belt needs to be connected with the tensioner loosened so that the tensioner can be used to take up any excess slack.
Do not over tighten as it could lead to a break.
Tim the belt needs to be loosened at the trolley with the door closed.
The belt should still have a little slack when the door is closed.
Brand new chamberlain garage door opener with excessive slack in belt drive.
But basically assuming the belt is attached with enough slack removed the tensioner bolt should be able to taske up the rest of the slack once tightened.
You may find that a loose garage door opener chain has less power to open and close your garage door or you may find that it is just hitting the door as it moves it and creating an annoying sound.
The information for tightening the belt is provided in the image below.