Hi crossleymarko,
The three screws are the High RPM adjust, the Low RPM adjust and the Idle adjust. The High RPM adjusts the top speed of the engine, lean = fast but hot running, Rich = slow but cool running. The Idle adjusts the engines idle RPM. Too slow and it will kill, too fast and the clutch will engage. The Low RPM adjustment is used to make a smooth transition from idle to high. You adjust it so when you hit the throttle the engine doesn't sputter and choke for a few seconds as it try's to transition to high RPM.
Properly adjusting a carb is a bit of an art. Experience is the key. The carburetor must be in good condition to make the engine run well. If the carburetor is dirty and in need of a rebuild you will never get it adjusted well. If you are not familiar with the procedure used to tune a carburetor you should consider taking the tool to a repair shop to have it tuned. You can quickly ruin an engine if you accidentally adjust it too lean even for a few seconds.
If the idle cannot be adjusted to that the clutch no longer engages then I would suspect a problem with the clutch itself. One of clutch pads is probably stuck.
I hope this helps you out. Good luck with your repair!
-UtahAV
-Mark