1981 Johnson 70 hp

[ToonerJim]

I'm the proud owner of a 1981 70 hp Johnson that has developed a serious vibration at 50% + rpm, under load. Motor sounds good at idle and at 25% a throttle. Vibration is considerably less if I raise on trim, but increases when trim is bottomed out. Hoping that I can get a little guidance before I start dismantling.

 

[orbanp]

Re: 1981 Johnson 70 hp

Hi Jim,

Welcome to the iBoats forum!

Have you hit something with your prop?
Check your prop, remove it, and see if the propshaft is bent.

Peter

 

[ToonerJim]

Re: 1981 Johnson 70 hp

Hi Peter,
thanks for the reply. Great forum -- lots of good info
I certainly did not feel anything. There are some nicks on the prop from previous hits in the shallows, but nothing that has bent the edges. I'm thinking that it may be the outer bearing on the prop shaft cause there seems to be some play, and a bit of oil leakage stain down the skag.

 

[orbanp]

Re: 1981 Johnson 70 hp

...
I'm thinking that it may be the outer bearing on the prop shaft cause there seems to be some play, and a bit of oil leakage stain down the skag.

Hi Jim,

That does not sound too good! The propshaft is supported by needle bearings at the back, and the shaft itself is the rolling surface of the bearing.
On this page there is a parts list for your motor, just pick your model number: http://www.marineengine.com/parts/johnson-evinrude-parts.php?year=1981&hp=70

Good luck, Peter

 

[ToonerJim]

Re: 1981 Johnson 70 hp

Hi Peter
Well, I think you were right with your first comment. I checked the shaft end and there is about 1/16" + run out. I'm supposing that may be enough to cause the vibration.

 

[ToonerJim]

Re: 1981 Johnson 70 hp

Hi Pete,
Thanks for the parts link -- sure helps

Cheers

Jim

 

[orbanp]

Re: 1981 Johnson 70 hp

Hi Peter
Well, I think you were right with your first comment. I checked the shaft end and there is about 1/16" + run out. I'm supposing that may be enough to cause the vibration.

Hi Jim,

It looks like you need to take that gear case apart. Put the shaft between points, and check the run-out with a dial indicator. You probably need to ask a machine shop to check that out for you. The machine shop even might be able to straighten that shaft out.
As a backup plan, start looking for a used replacement gear case, or a blown motor with a good leg, as parts seem to be expensive.

Peter