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.
Tighten garage door belt.
Step 1 closing the garage door before beginning use the garage door opener to close the door and fasten it.
In most cases all you have to do is adjust the opener belt of the belt drive tightening it to ensure smooth running.
The only thing you need to do with a belt driven opener is to adjust the belt occasionally.
The belt needs to be connected with the tensioner loosened so that the tensioner can be used to take up any excess slack.
All you need to get started is a step ladder a 7 16 inch wrench and a flat head screwdriver.
Loosen the adjuster three turns counterclockwise if the door raised without.
Brand new chamberlain garage door opener with excessive slack in belt drive.
The specific procedure should be in the manual for your unit.
This is a very simple job to do yourself in fact all you need to do is follow this simple 6 step guide.