You have 2 screws that hold your clutch on to the end of the crank. Those screws are probably backing out and your clutch is loose. Your engine is supposed to rev freely to 8k rpm for your clutch to engage. With a load on it, your clutch is engaging too early and its bogging out. The engine needs to spool up freely to build rpms.
thats why I suggested you rev it with no load on the tires. If it revs half way decent, then that's your problem