1980 70 hp Johnson problem at high rpm

[rlowhorn]

Just got an old boat into water. The motor is 70 hp and seems to start and run good, except at high rpm. Something happens that I can only describe as it seems to loose contact with water. The motor revs up until I back off the gas. Then it runs okay again. Any thoughts would help.

 

[wilde1j]

Re: 1980 70 hp Johnson problem at high rpm

Likely spun prop hub ... mark inner and outer hubs with an indelible marker, run under load and recheck. If marks have moved relative to each other, hub is spun. Take prop to prop shop for rehubbing.

 

[bktheking]

Re: 1980 70 hp Johnson problem at high rpm

Take a pic of the motor from the side of the boat, suspect it's set to the wrong tilt hole, unless it has power tilt and trim.

 

[bktheking]

Re: 1980 70 hp Johnson problem at high rpm

Or a spun prop LOL

 

[rlowhorn]

Re: 1980 70 hp Johnson problem at high rpm

Likely spun prop hub ... mark inner and outer hubs with an indelible marker, run under load and recheck. If marks have moved relative to each other, hub is spun. Take prop to prop shop for rehubbing.

What does it mean to be rehubbed? I pulled the prop off and it appears that the shaft teeth and that on the prop are still in pretty good shape.

 

[bktheking]

Re: 1980 70 hp Johnson problem at high rpm

The teeth being in tact is good but doesn't tell you much.

Read this link, first part about spun props.

http://www.mercstuff.com/spunprop.htm

It's the rubber between the splines and the prop itself that breaks away, modern props don't use shearpins, they use rubber instead. The link also explains how to check it, Wilde1j Explained it as well.