94 yamaha 70hp idles great but will not Rev up

[B.B.Upch]

Hello guys I'm trying to work on my dads boat and I have found a lot of help on here for mine so hoping for answers. So here is what he has.
a 94 yamaha 70hp running great for 15 trips or so then went to take it out. It started & idled great but when we put it in gear & throttled it up it did nothing. We could hear/feel it go into gear but it stayed at idle? We got it home & checked the prop & it spun freely. We hooked it up to elephant ears & it ran great. Reved up great & went into gear & the throttle spun just like its suppose to. My dad took it out about 3 times & it worked perfect. Then today he took it out & it was doing the same thing. Got it home & hooked it up to water hose & works fine.
so from some reason it starts & runs great & works put of water but in water (under a load) it just idles & will not REV up.

 

[rejesterd]

If an engine is in-gear, you don't need to listen to or feel anything. Either you're moving forward (i.e. you're actually in-gear) or you're not (i.e. you're actually not in-gear). If you could clarify that point, we can probably help.

 

[B.B.Upch]

It goes into gear & barely moves forward but the engine bogs down & no matter how much throttle you give it then engine never gets above an idle... this only happens in water (under a load)... out of water it seems to run fine

 

[rejesterd]

I would replace the fuel filter and see how it goes then. Also, make sure the prop does not spin freely when it's in forward or reverse gear. If it does spin by-hand when it's in-gear, that's wrong.