With the gas drained, battery removed, and the sparkplug disconnected or removed, stand the mower on its tail on a hard surface level area; that's what the bumper bars on the back are designed for.
Check the drive platter on the engine crankshaft for grease or oil. Clean it if needed. Also clean the rubber faced drive disc on the transaxle input shaft.
If cleaning it does not help, or if the rubber faced drive disc shows obvious wear or damage, replace it. The rubber is about 5mm thick on a new drive disc. If it is worn to less than 3mm, I would replace it with a new one.
Unless someone has had it apart or monkeyed with it, you should not need to change any adjustments, just clean or replace the friction drive components. If you do need to make adjustments, better buy a service manual or pay a professional mechanic to do it.
haywiremechanic, June 2009