To be a little more specific, jack up the rear tire with what ever you have to jack it up with. Make sure it is stable. Rotate rear wheel, and shift into 1st. Rotate or bump the wheel, and shift into neutral. Rotate wheel again, and shift into 2nd. Bump the wheel forward, while shifting into 3rd. if it will not shift, make sure the shifter lever is tight. If you have a 75 or 76, and all the cross over stuff, make sure the lever on the shift shaft is tight on the shaft(behind the cam cover(remove the kicker cover, or sprocket cover and you will see it). If you still have no 3 or 4, you may have internal problems, and that is a whole new story. Also, check your clutch adjustment. The higher the gear, the less torque it takes to make it slip.