1995 Johnson 70hp zero compression

[ChrisMA]

Just bought a boat today that has a Johnson 1995 70hp 3 cylinder engine. The engine starts right up but runs really rough. I had my compression tester in the car, but decided to leave it and low-ball the seller. Tonight I did the compression test and two of the cylinders are 100 psi and the center cylinder is 0 psi, no compression at all!

I am not too upset since I got a good deal, but now I need to figure out how to fix it. I ran out of daylight today, but will attempt a leak down test tomorrow. How is it possible to have zero compression? Could it be something as simple as a stuck reed valve or something as bad as a cracked block?

I would assume the next step is to take off the head and look at the cylinder wall and post a picture here. After that should I take it to a machine shop for honing then order a oversized piston kit? Can a first time engine rebuilder easily hone a cylinder accurately at home?

Any recommendations for oversized piston kit manufacturers?

The factory service manual is already in the mail.

Chris

 

[Daviet]

Re: 1995 Johnson 70hp zero compression

First thing to do is to pull the cylinder head and find the problem. Maybe you will get luckey and have a blown head gasket, easy fix.

 

[ChrisMA]

Re: 1995 Johnson 70hp zero compression

Thanks for the quick reply, I forgot to add that this boat has sat for 2 years or so.

Will the leakdown test tell me anything more? Most diagnostics I see online point to what happens with a 4 stroke.

Chris

 

[Daviet]

Re: 1995 Johnson 70hp zero compression

With zero compression I don't think a leak down test will tell you anything.
Pull the head and have a look.

 

[ChrisMA]

Re: 1995 Johnson 70hp zero compression

Great thanks I will check it out tomorrow. Since I do not have my shop manual yet, is there a proper way to unbolt the head in regards to a pattern or reducing the torque in steps?

Chris

 

[Daviet]

Re: 1995 Johnson 70hp zero compression

Not when taking the head off. There is a torque sequience when putting the head back on.

 

[Dhadley]

Re: 1995 Johnson 70hp zero compression

You can unbolt the head bolts in any order you like. On a 56 cid motor the center cylinder runs the hottest. Zero compression indicates a major failure, most likely a hole in the piston.

What spark plugs were in it?

 

[wilde1j]

Re: 1995 Johnson 70hp zero compression

Examine the head, gasket and block carefully, looking for leak paths. However, you may find catastrophic piston/cylinder damage. If you're thinking of possibly rebuilding, you want to determine the cause of the failure, or it will happen again.

 

[ChrisMA]

Re: 1995 Johnson 70hp zero compression

It has CHAMPION QL78YC plugs in it which are the right ones. Is it possible that the rings are stuck from sitting a while? I have no issue pulling the head, but then I need a new gasket to put it back together. All 3 plugs look pretty good which is strange. I did spray some seafoam in the carb and in the spark plug hole tonight. Maybe I will quickly try a test again in the morning before I pull the head.

I do plan on rebuilding it, that is part of the fun of an old boat.

Can I pull the powerhead while leaving the mid and lower unit on the boat (assuming the head is rebuildable)?

Chris

 

[Daviet]

Re: 1995 Johnson 70hp zero compression

Yes, you can pull the powerhead while the engine is mounted to the boat.

 

[ChrisMA]

Re: 1995 Johnson 70hp zero compression

Well this is not going to clear up with a pint of seafoam.

It looks like they are the original pistons 435547. The top and bottom pistons are 115 PSI when the engine is warm.

What is the next step? Strip it down? Normally do you bore and replace all pistons at the same time? There is no cross hatching on any of the cylinder walls.

Does this damage look slight enough that a new piston and ring with a honing will fix it?

Chris

 

[Dhadley]

Re: 1995 Johnson 70hp zero compression

When you have a piston burned by the exhaust port like that it's most likely an ignition problem. Simply changing the piston will fix the damage but will not fix what caused the damage. Not fixing the cause may result in another damaged piston.

Now, the reason the spark plugs look almost new is because they probably are. Lots of times when a motor starts running bad someone will put in new plugs thinking that's the problem. In this case it wasn't. The root cause may be as simple as finding out someone had the wrong plugs in it before. Probably not but it could be.

As for boring, you'll know that once you get it apart and measure the cylinders.

 

[ChrisMA]

Re: 1995 Johnson 70hp zero compression

When you have a piston burned by the exhaust port like that it's most likely an ignition problem. Simply changing the piston will fix the damage but will not fix what caused the damage. Not fixing the cause may result in another damaged piston.

Now, the reason the spark plugs look almost new is because they probably are. Lots of times when a motor starts running bad someone will put in new plugs thinking that's the problem. In this case it wasn't. The root cause may be as simple as finding out someone had the wrong plugs in it before. Probably not but it could be.

As for boring, you'll know that once you get it apart and measure the cylinders.

Interestingly enough there was an ignition module in the boat, maybe the module was bad causing this damage? Can you adjust the timing on this motor, could it have burnt though due to improper timing? I figure rebuilding from scratch and readjusting everything should fix all the issues? Are you suggesting more troubleshooting?

Another thing I noticed is there is barely any water coming out of the tell tale, could poor cooling caused this? Obviously I plan on replacing the impeller, wear plate, etc when I rebuild.

Chris

 

[ChrisMA]

Re: 1995 Johnson 70hp zero compression

Wow this engine has a lot of parts. I have it half disassembled right now and was able to successfully get the flywheel off which appears to usually be quite a feat.

How do I remove the lower cowl? It appears that that needs to come off to remove the block.

I am not getting the manual until Tuesday.

Chris