slipping clutch when motor "on the pipe" can only be the result of the following:
1-incorrect/incompatible primary/trans fluid which contaminates the friction plates causing slippage
2-incorect cable/clutch pack adjustment. did you adjust the clutch pack tension screw correctly. you turn it OUT....COUNTER-CLOCKWISE till just a bit snug....then IN...CLOCKWISE approx. 1/2 turn and secure with spring/lock cap nut.
3-diaphragm spring end of its lifespan and needs replaced.