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1991 Mercy 115 hp carbureted...good motor?

BigGameBalls

New member
My brother is considering buying a boat with a 1991 carbureted 115 hp Merc on it. Are these good motors? Are there any common problems with them? What should the compression be to be considered "good". The seller has claimed to have had a compression check done recently. Anything specific he should look for?

The boat is located out of state so I'm just trying to help him do as much research before he drives 600 miles to buy it if everything checks out.
 
Re: 1991 Mercy 115 carbureted...good motor?

Compression testers are not all that accurate. One could tell you a cylinder reads 100psi while another reads 120. The most important thing (assuming the engine runs) is that the variance between cylinders isn't too great when measured with the same tester. If one cylinder reads 118 and another 103, you're may be looking at a problem.
 
Re: 1991 Mercy 115 carbureted...good motor?

The factory compression spec on these was 150-180 psi. Below 120 is considered "shot".

But as Jake notes, unless the gauge is calibrated properly, it's the balance between cylinders that is more important. On this model I would be looking for them all to fall within about 15 pounds of each other.

I liked Merc's inline-4 (2+2) models - the 115/125 in particular.

They idle on 2 cylinders and the number 3/4 kick in around the 1800 rpm mark.

Those not familiar with the operation are sometimes concerned/surprised when the other two cylinders fire up - it can be a little "violent" when they transition (going to all 4 or back down to 2). But once you are comfortable with that - which is normal - you will then be able to recognize when something is really "wrong".

These are no more prone to issues than just about any outboard out there and are particularly well suited to fish/ski boats in the 16-18 foot class when you want a little more kick than you get from 3 cylinder models (such as the 90) but don't quite want to move up to the 135 horse V-6's..
 
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