well, if you have tried adjustments to the grub screw, And if you have had this problem since last time you put the tranny together, it is possible that you have the clutch in backwards. OR, the second gear spur could be stripped on the inside where the clutch arm grabs. OR, your engine may not be getting up to the proper RPMs, but generally, like I said, you can get the engien up to full rpms by holding it off the ground and giving it throttle (no resistance).