Lower unit issues?

[bsutravis]

I'm dealing with a '78 Mercury 115hp:

A couple months ago I spun the hub of my prop. I replaced the prop with my spare, and thought I was good to go. A few weeks ago I took the boat back out to the lake and I'm still having a major issue. I can only give about 25% throttle before the RPM's over-rev. Up till that point the boat runs fine, but once I hit that spot with the throttle it revs up.

Right now I'm freaking out thinking something major is up with the lower unit, but I'm not even sure how to begin to troubleshoot the problem. I suppose I need to drain the L/U fluid and check for metal / water. Then what? The Selok manual looks intimidating in terms of tearing into the gear case, especially when I'm not sure what I would be looking for in terms of what is wrong. Are there any obvious things I need to be looking for to troubleshoot this? Also, is it better to try to fix a broken piece in the L/U or just look for a used or rebuilt L/U?

Any advice is appreciated.

 

[Moody Blue]

Re: Lower unit issues?

First thing as you mentioned is drain the oil and see what comes out.

Does it do the same thing in fwd and rev?

Could be your spare prop is also spun. Take a marker (waterproof) and draw a line across the prop body and rear hub face. Take the boat for a spin (rev it up) then bring it in and check that the line you drew is still in-line. If not, then your prop is shot.

Would not recommend tearing into the lower unit unless you've got the right tools and a good manual.

 

[Chris1956]

Re: Lower unit issues?

You are on the right track. Drain the gearoil and inspect for water and pieces of metal. If you find metal, you have major issues. If you find a little water, that may or may not be an issue.

Are you sure your prop does not have a spun hub?