Yamaha 25 astern problem

[nickos_w]

Hi,

I have a yamaha 25 autolube 2 stroke I recently acquired on a small rib. The boat has a console with yamaha throttle unit. When going into astern at sea at any real power the gears crunch horribly. Out of the water no problems seem apparent at all, and the prop spins as expected, as if the load on the engine makes the gear slip?

Any thoughts would be really appreciated

Many thanks !