Mid 80s 70hp Cylinder Compression

[scrampbell]

Hi all,

I'm thinking of buying my first boat and I need some motor advice. I've been told that one of the most important factors is the condition of the motor. That being said, the seller initially said the compression was good on this older 70hp Johnson (I'm guessing early to mid 80s by the picture), but when I mentioned that I would want to check the compression again before the final sale, he backpedaled a little bit...

He said he had the compression checked last year and that the compression was good on #1 and #3 cylinders, but that they either couldn't get to or couldn't get a reading from #2. I wasn't real clear on exactly what he was saying except that it sounded like bs. So...

Question #1: How well would the motor serve me if in fact 2 out of the 3 cylinders are good right now? Could I expect it to perform as about a 45hp or so? Could I use it as is?

Question #2: Are there specific questions that I should ask to asses how broken the cylinder #2 is? I wouldn't mind working on it if there were a way to determine if it were fixable.

Thanks!

 

[BKS72]

Re: Mid 80s 70hp Cylinder Compression

Compression is one of the key factors in motor "health" - low compression on a cylinder could be as minor as a head gasket on the low end and as major as a holed piston or scored cylinder wall (or both) on the high end.

Parts for outboards are quite a bit more expensive than they are for cars and depending on what's causing the low compression you could need lots of parts and machine work.

As far as using it as is, if it's a minor problem you'll just turn it into a major one and if it's a major one at some point the motor will most likely seize. Without pulling the head it's hard to tell which kind of problem the motor has.

From my limited experience I would advise bringing a compression gauge with you and checking all three yourself. If they're not all above 100psi and within 10% of each other I would either pay very little for the motor if I wanted a project or keep looking if I wanted a boat to use right away. Not many questions you could ask - if they seller was in fact prevaricating about the results of the compression check he did I wouldn't put much value in other info provided......

Good luck!

Branden

 

[kenmyfam]

Re: Mid 80s 70hp Cylinder Compression

Hi all,

I'm thinking of buying my first boat and I need some motor advice. I've been told that one of the most important factors is the condition of the motor. That being said, the seller initially said the compression was good on this older 70hp Johnson (I'm guessing early to mid 80s by the picture), but when I mentioned that I would want to check the compression again before the final sale, he backpedaled a little bit...

He said he had the compression checked last year and that the compression was good on #1 and #3 cylinders, but that they either couldn't get to or couldn't get a reading from #2. I wasn't real clear on exactly what he was saying except that it sounded like bs. So...

Question #1: How well would the motor serve me if in fact 2 out of the 3 cylinders are good right now? Could I expect it to perform as about a 45hp or so? Could I use it as is?

Question #2: Are there specific questions that I should ask to asses how broken the cylinder #2 is? I wouldn't mind working on it if there were a way to determine if it were fixable.

Thanks!

To answer question 1. Two Out Of Three Ain't Bad only works in the song by Meatloaf.
To answer question 2. If you can not hear it running on muffs, have a test run on the water or get a warranty then the motor is worth very little in my opinion. You could end up putting a lot of time, money and effort into getting it running correctly. Would you buy a car that ran on two thirds of it's cylinders correctly and just go slower ???

 

[scrampbell]

Re: Mid 80s 70hp Cylinder Compression

Thanks for the comments guys!

After talking to the seller a little bit more I'm thinking of taking the boat for a test run and check the compression myself. Is there anything about a 1985 Johnson 70hp that I need to know to get an accurate compression reading off of all cylinders or will it be pretty standard?

Also, I've read other places and BKS70 says in his comment that the compression should be above 100psi and within 10% of each other... With an older motor like this one, would it be acceptable for the compression to be in the mid 80s, but within 10% of each other? Could this just be a sign of wear with age and could I expect that some standard maintenance would do the trick (i.e. new head gaskets and rings)? Or would this motor still be a major project if I found the compression to be below 100 but within 10% as the seller has said recently?

Thanks again!

 

[Dhadley]

Re: Mid 80s 70hp Cylinder Compression

OMC never gave any specific compression numbers for that motor. As long as they are all within 15 lbs of each other it should be OK. A compression test is a basic test that gives some indication of how healthy the motor is. A leakdown test will be benifical also.

 

[HybridMX6]

Re: Mid 80s 70hp Cylinder Compression

Agreed. Do the comp test if you can, no reason you can't. I'd also make sure to check the lower unit oil. Just remove the bottom drain screw and see what comes out. Put a small container under it to catch whatever it is, and take a closer look at that as well. If it's nice good looking oil, either it's in good shape or he just changed it to avoid people noticing it.
Water test is the best thing you can do. If the lower leaks, you will know it after a brief run on the water. Running on muffs won't tell you anything about the condition of the LU.

 

[kenmyfam]

Re: Mid 80s 70hp Cylinder Compression

Also compression testers vary in their accuracy as well. 2 testers can mean 2 different readings.

 

[scrampbell]

Re: Mid 80s 70hp Cylinder Compression

My sincere thanks to you all. I'm going to take it for about a 20 minute test spin tomorrow and will check the compression afterward in all cylinders. I won't do the leakdown test as I don't have the equipment. If the test run goes well (no oil in the water, no odd rumbles from the motor, no sinking!) and the compression in all cylinders are within 15psi of each other, I'll send pics of my new boat!

Now... to figure out if the guy posting about getting the 70hp he's selling in working order is the same guy I'm dealing with!