Possible 120 coupler problem.

I have a 1969 120 hp Mercruiser that I have been attempting to bring back to life. I seem to finally have the engine running ok and took it out today to find out what is going to be next on the repair list. I maxed the throttle out and what I heard was a loud quick squeal, momentary loss of power and the engine revved up without a load. I felt the coupler- there was a slight smell of burnt rubber, it wasn't warm, but I had only been moving on the water at slow speed for just a couple of minutes.

I tried two differently pitched marked props to rule out a propeller problem. I didn't gun it again for fear of getting stuck on a desolate lake but could hear a high pitched whine at half throttle while there was a obvious loss of power. It felt as though the propeller would stop spinning for a second or two every few feet. It would move forward while idling in gear without a problem.

The, propellers, u-joints, splines, gimble bearing all look real good, cables seem to work good and it shifts fine.

With my limited experience with such things I am guessing the coupler is shot. Would my guess sound about right?

Re: Possible 120 coupler problem.

the "wasn't warm" part seems odd. Both couplers I had fail were smoking hot, but I suppose if the rubber just rotted, it could slip without much resistance. Sounds more like a problem inside the drive to me. I would drain fluids and inspect.
For further coupler diagnosis, I would stick my head back there while someone else drives. maybe give it something to get hot about (watching engine temp of course)

Re: Possible 120 coupler problem.

Thanks for the reply, I hope it's not the outdrive- I haven't got them figured out yet. It has what appears to be new u-joints and the oil is ok, no water anyway. Manually rotating the prop in both gears works the way it should, however while standing on the prop while in gear I can't get it to twist in the coupler- but on the other hand I don't weigh much either.

I was worried about getting stuck out at the lake so I didn't want to run it very hard.
It's a very isolated lake in western Kansas. As far as I could tell I was the only one out there today- 7000 acre lake to myself, kind of disconcerting for a person who doesn't like to row.