P. Dave...

The prop hub is slipping. The shear pin on that engine actually connects
the prop shaft to a separate shaft/steel hub which is pressed into a rubber
hub/bushing which is then pressed into the propeller housing. It all looks
like one piece as mounted on the engine, but when you take the plastic prop
nut off with the cotter pin, and then slide the shear pin out and prop off,
it will become obvious on the back side (fore facing) as to the
construction.

Get a new hub pressed in at a prop shop, and get a spare prop too.

Good Luck!

Rob
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"Booms" wrote in message
news:Rj27b.9866$J6.639@pd7tw3no...
I am a boating newbie and the new owner of a 1970-something Johnson 40hp
outboard. I have had it out for maybe 8 hours so far without any
problems.
The other day it started acting up. When I push the throttle forward, the
boat moves, but then the engine just rev's up and the boat doesn't go
anywhere. The closest comparison I can make is that it seems like a
clutch
is slipping.

I'd like to repair the engine myself. Where should I start looking?

Thanks for any info,

Pyro Dave.