More than likely you have a problem with the centrifugal clutch, but either one is going to require taking the front cover off the engine to determine what is wrong.
You can somewhat test the manual clutch (the one with disks) by riding the machine and releasing and engaging the clutch using the gear shift lever. As you press down or lift up on the lever, it releases the clutch to facilitate shifting. If you press the lever gently, it will release the clutch without shifting gears. If you barely press the lever and the clutch slips madly, try readjusting the clutch for more play before it releases.
It's rare to have a problem with the manual clutch. About the only way it happens is if the adjustment was too tight and the clutch was slipping or the springs came loose, broke or otherwise lost tension causing the clutch to slip. That causes overheating and the clutch will burn out. It's more likely the centrifugal clutch is not locking up due to breakage or use of improper oil. Only fix is to get it apart and find out what is causing the problem. Order a gasket and the two special nuts for the clutches.
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