Late 70s Mercury 20 horse 2 stroke Forward problems

[tolbertn]

Alright Guys I think I have a problem
I think I have a late 70's long shaft 20 horse mercury 2 stroke. The motor runs strong and had been now for about 50 hours or so on our pontoon. Just recently we were coming out of the channel and as I throttled up, we seemed to just slip out of gear. The weird thing is that it was still in forward and the prop was still spinning fast enough to keep us at a trolling speed but when trying to throttle again it would just increase RPM and not going any faster than the slow trolling speed.

Is my lower unit toast??

I originally thought that maybe the splines on my prop might have just spun off and a new prop would be all that was needed but after checking they were fine.

Troubleshooting advice would be extremely helpful.

 

[twindad01]

Re: Late 70s Mercury 20 horse 2 stroke Forward problems

I had a similar problem and found it to be debrea between motor and transom caused a flow problem with the water and prop was sucking air could also be a spun prop hub

 

[Texasmark]

Re: Late 70s Mercury 20 horse 2 stroke Forward problems

I had a similar problem and found it to be debrea between motor and transom caused a flow problem with the water and prop was sucking air could also be a spun prop hub

Sounds like the spun prop but not being a toon guy not familier with twin's other comment, but surely a possibility. Take the prop off and using a magic marker on the rear of the prop, scribe a line from the spline all the way out to the edge of the prop hub. Put it back on and run it till is does this again. Remove it and inspect your line. If still straight, suspect debris. If misaligned, get a new hub/prop.

Mark

 

[tolbertn]

Re: Late 70s Mercury 20 horse 2 stroke Forward problems

Texasark,
I am going to try the line idea, and adjusting the motor so it is sitting straighter so the possible air problem gets eliminated. Thanks for the advice guys and ill let you know how it goes.