Wondering if anyone can make some suggestions of where to start
looking. My 71 20hp johnson sometimes momentarily jumps out of gear...
usually it does it for literally a second and it almost feels like the
boat has hit something in the water but it always keeps moving and has
never caused a significant problem. It only ever happens as the boat is
building speed, when under high load, once the boat is up on the plane
then it never happens. Today, however, it did it again, but it didn't
go back into gear until i reduced throttle, put it in neutral and then
back into forward gear again. After that it was ok again.

Each time it happens the gear lever on the side of the engine does not
move at all so it is something internal. Obviously ther is something
slipping, etc. I have rebuilt many car and motorbike engines, however
this is my first boat and i don't know the internals on this engine
yet. Could anyone advise me on where to start looking before i pull it
apart. Are there any common problems with the engine?

Thanks Dave.