1981 Johnson 70HP No Compression

[BigBankWA]

I was given an 1981 70HP Johnson that had an exhaust manifold leak and had been "put up" three years ago. I fixed the exhaust (broken bolts) and ran a compression test and it shows at almost nothing...like 15lbs is all.

I am relatively new to outboards...and have just enough knowledge to be dangerous. What possible issues could this motor have that would cause it to have almost ZERO compression in all 3-cylinders. I know I am probably looking at a rebuild, just was wondering what the possible causes are for this. One cylinder I could see...BUT ALL THREE?

 

[ezeke]

Re: 1981 Johnson 70HP No Compression

Probable that someone removed the cylinder head for inspection and did not replace it with a new head gasket and torque it properly.

Are you using a screw in tester that will hold the reading until you release it?

 

[a70eliminator]

Re: 1981 Johnson 70HP No Compression

Well at least they're consitant with being 10% of each other. I would first verify my compression gauge isn't kaput. It would have to be a mojor crack in the block to effect all 3.

 

[BigBankWA]

Re: 1981 Johnson 70HP No Compression

Well back in from the garage and looking at the responses so far. Thanks for your input. I really do appreciate it. Wish someone with some knowlege was here...would definately be worth a cold beer or 24 of them to talk motors with someone...alas...my outboard guy is in Seiku fishing Halibut.

I didn't believe my 20 yr old, press into the socket type, compression tester either...so I went and bought a brand new one...screw in type so I dont have to ask the wife or 6-yr old to turn the key anymore...and same readings under 20 lbs....

Story from previous owner was motor was running fine on a trip, suddenly would not go above idle...give it gas, dies. He figured it was carbureation and had all 3 carbs rebuilt (they are so clean you could eat off them). But still had the same problem.

Took it to a mechanic and they diagnosed an exhaust manifold gasket leak. Said it was allowing water into the number one piston as soon as you gave it any RPM's and choking out motor. They cleaned out the cylinders etc with oil and he towed it home. He started repair, broke 2 bolts, and got distracted. Sat for 2+ years.

I spent the last 2 weeks taking care of the exhuast issue, the bolts were a nightmare but I managed to get them all out and save the threads...and now I find no compression.

Is a cracked block really the only other thing that could cause this? I know its practically impossible to diagnose anything over the computer 100%...I just would like to have an idea on what items to pay particular attention to when dismantling.

As far as the head not being torqued down properly, not the case here. LOL. Those bolts are in to specs, and from the looks of the gasket have never been cracked.

Previous owner was original owner...friend of family...says never rebuilt. Have no reason to doubt anything he has told me...like I said, motor was free to me...except for all the drill bits, easy outs, PB Blaster, WD40, and tools I have had to buy. )

Seems like motor is a solid foundation and matches up to my 1984 Tiderunner Runabout like a dream. Salmon season is upon us, and I really want to get this motor running. Just not the funds to have the work done, but the connections to get the parts and take a run at it...if its worth rebuilding.

 

[ezeke]

Re: 1981 Johnson 70HP No Compression

Did you take the cylinder head off for inspection? You should be able to see the cylinder damage clearly. Post some pictures of what you find if you can.

 

[airmail]

Re: 1981 Johnson 70HP No Compression

Did you open the throttle while doing the compression test? Did you do a wet compression test? If someone was having a hard time starting this motor they could have wiped the oil off the cyls. I had this happen to a vehicle I bought. Very low compression in all cyls. I did a wet test and all the cyls came up to 180 psi. I have been driving it for 6 years now.

Does the motor crank over fast with the plugs out?

 

[reelfishin]

Re: 1981 Johnson 70HP No Compression

I just picked up a 15 hp motor which had only 20 psi on both cylinders, it had sat for several years but ran fine when put away. I got it cheap assuming it needed a rebuild. I shot some light oil in each cylinder, cranked it a bit and tested the compression and got 135 on both cylinders. It runs fine, needed nothing but a new impeller.

You could have corrosion in the cylinders, stuck rings, etc. too if the motor wasn't stored properly or if water was left sitting in the cylinders.
I'd be likely to pull the head to get a good look just to make sure all was ok first if a little oil in the cylinders don't bring back the compression.