loose lower unit on 3 hp Gamefisher

[kfa4303]

Hello all. So I just got my little 3 hp Gamefisher up and running. New gear oil, new gas, new spark plug. The little guy runs like a champ! The only problem is that the lower unit won't stay clamped to the vert prop shaft tube. I have tightened the built in clamping mech as far as it can go, but after I run the motor a bit the vibrations cause the lower unit to shimmy down and inch or so, which in turn causes the vert drive shaft to come out of the centrifugal clutch on the underside of the engine therefore prop won't spin even though motor goes like mad. It all held together well before I started working on it due to gunk, dirt, friction, but now that everything is nice and clean it's as if there isn't enough friction btwn. the clamp and the prop shaft tube, or something. Any ideas how to fix? Many thanks!