1981 Evinrude 7.5HP - Drive starts to slip at higher RPM's

Viking59

Cadet
Joined
Sep 2, 2012
Messages
17
I have an 81 Evinrude 7.5HP short shaft motor. When you open up the throttle about half way the motor over revs and loses propulsion. When you come off the gas it seems to pick up propulsion again with no issues.

I have taken the leg off for inspection and the bottom end looks good. There were no chunks of metal or teeth missing at the top of the drive shaft. Is there more than one issue that this could be or is this definitely an issue with the base of the crank that the top of the drive shaft connects to? If that is the issue, I just don't understand why the motor would have propulsion at low RPM? Wouldn't it just slip all the time?

I have also checked the prop hub and that is not the problem.

Can anyone recommend a resource that would guide me through taking off the upper part of the leg so that I can inspect the base of the crank?

Thanks all!
 

Chinewalker

Fleet Admiral
Joined
Aug 19, 2001
Messages
8,902
Re: 1981 Evinrude 7.5HP - Drive starts to slip at higher RPM's

There's pretty much only three things it could be.
1 - the prop hub, which you've already checked. How, exactly, did you check it?
2 - the lower unit is too short for the transom, which, if the motor has worked well for you previously, likely isn't the issue.
3 - the driveshaft is slipping in the crankshaft, which is a possibility as these motors seem to be susceptible to crankshaft rust. If you drop the lower unit (three bolts) and look at the upper splines, do they show any evidence of wear?
 

Viking59

Cadet
Joined
Sep 2, 2012
Messages
17
Re: 1981 Evinrude 7.5HP - Drive starts to slip at higher RPM's

There's pretty much only three things it could be.
1 - the prop hub, which you've already checked. How, exactly, did you check it?
2 - the lower unit is too short for the transom, which, if the motor has worked well for you previously, likely isn't the issue.
3 - the driveshaft is slipping in the crankshaft, which is a possibility as these motors seem to be susceptible to crankshaft rust. If you drop the lower unit (three bolts) and look at the upper splines, do they show any evidence of wear?

Thanks for your reply.

I took the proper off and drew a line across the hub and prop with a jiffy marker. Nothing was off after I revved the motor up.

This sounds stupid but I didn't even think about the transom height, although I don't think that is the issue. I will take the boat out again this weekend and check. The transom is relative low in my 14' aluminum boat.

I'm of course most concerned about issues with the base of crank shaft. When I took the drive shaft off I would have expected to find signs of wear and metal shavings at the top of the drive shaft. It looked clean to me and wasn't rusty.

Thanks
 
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