These videos below will explain how to properly adjust your clutch on your motorized bicycle:
Adjusting Your New Motorized Bike's Carburetor & Clutch
How To Adjust Your 2-Stroke Clutch - 48cc 50cc 66cc 80cc -Motorized Bicycle
You can actually adjust your clutch before installing the engine on the bike. To check to see if you need to adjust your clutch, push the clutch arm down towards the engine and attempt to spin the engine sprocket with your finger. Naturally, there will be some resistance on the clutch arm. Use your spark plug removal tool to help push the clutch arm down easily. If you are unable to move the engine sprocket freely, it means that you will have to adjust your clutch.
The following instructions should help you adjust the clutch properly and determine if there is an issue with your engine:
• First, remove the gear case cover on the engine.
• You should see two gears, one large gear and one small. On the large gear, there is a wing (wheel) nut in the middle of it with a Phillips screw holding it in place.
• Unscrew the Phillips screw.
• Remove the wing nut by turning it counter-clockwise using a chisel and hammer (or flat head screwdriver). Once you get 1-2 full turns, you can pull the clutch arm in and hold the clutch cover down to remove the wing nut easier and faster.
• Once the wing nut is removed, remove the clutch cover.
• Remove the spring under the clutch cover.
• Three pegs are now exposed. Rotate the pegs to make sure they spin freely. Some resistance is normal. If you are unable to move the pegs freely with your hands, gently assist rotation by lightly tapping the pegs with a hammer.
• Reinstall the spring and then clutch cover.
• Reinstall the clutch wing nut hand tight. The way the clutch works is that the clutch arm pushes the clutch cover away from the clutch pads, freeing the drive chain so you are able to pedal your bike (disengage). When the clutch arm is not pulled in, the clutch cover touches the clutch pads (engage). You must adjust the wing nut so the clutch cover moves 1/8" when you disengage the motor properly. If the displacement is more than 1/8", then your clutch will slip when attempting to start the engine. If it is less than 1/8", it will be too tight to disengage. Make sure you adjust it just right.
• Then reinstall the gear case cover.