Low Compression on one cylinder........

[mrcj001]

1975 Johnson 135 HP

Been having idle and hard start issues, started late spring kept getting worst. checked compression

Cylinders 1,2,3 are 125,115,115...Cylinder #4 is 40lbs (Cold engine)

No knocking or other unuasual engine noise.

Is there away to tell if its a head gasket or the piston compression ring?

If I pull the head, what do I look for ?

 

[whalerfreak22]

Re: Low Compression on one cylinder........

i remember when my 1992 112 johnson lost a cylinder it had the same readings as you but anyway pull the head you never know until you do.when you pull it look for scratches/scoring on the cylinder walls and piston.let us know what you find.

 

[Mntom]

Re: Low Compression on one cylinder........

You can try squirting a little oil around the piston then take a compression reading. If it comes up then you have a ring/piston issue and if compression does not increase then it most likely is a headgasket. When you pull the head look at the gasket and you should have no problem finding the leak (if that is where it is).

 

[kenmyfam]

Re: Low Compression on one cylinder........

Give it a good decarb and check again. Could be a sticky ring.

 

[emdsapmgr]

Re: Low Compression on one cylinder........

Pull the head on that side and check the head gasekt seal ring around the low compression cylinder. Possible that seal ring is shot and is leaking compression. If so, it's easy to replace the head gasket. While you have the head off, check the cylinder walls as whaler mentioned. Also try to inspec the top ring on the piston. You should be able to see the ring with a flashlight-check to make sure it is in one piece. When new, that engine should have had over 130 lbs compression on each cyl.

 

[mrcj001]

Re: Low Compression on one cylinder........

You can try squirting a little oil around the piston then take a compression reading. If it comes up then you have a ring/piston issue and if compression does not increase then it most likely is a headgasket. When you pull the head look at the gasket and you should have no problem finding the leak (if that is where it is).

That test makes since.....

What kind of oil is best to use?

How long to let it set?

Or should I use Seafoam in the cylinder?

I`m gonna fool with it tonite....................

I really hope it is a simple head gasket, this motor has too much age to do a total rebuild. I`ve looked around and can get mid 80`s engines 70-85hp with T&T for $1200-$1500. I just hate to spend that much money on this old boat.

 

[ezeke]

Re: Low Compression on one cylinder........

If you have a broken ring, continuing to run the engine will do more damage.

Removing the cylinder head for inspection requires removing ten screws and replacing one cylinder head gasket.

 

[mrcj001]

Re: Low Compression on one cylinder........

UPDATE:

I didnt have time to remove the head tonite,late gittin home from work.

Before I left this morning , I put some 2 cycle oil in the cylinder and let it set all day.

I ran the engine for about 10 min. The compression came up to 80 lbs.

I then ran about 1 gal of fuel with 16 oz. of Seafoam thru it. Compression is now up to 90 lbs.

1,2,3, 125,125,120 cyl #4 90

Also, after running the engine, when I remove the spark plug on #4, it seems to be more "steam" or "vapor" coming out of the cylinder thn the others.


Am I on the right track?

Any other tips?

 

[kenmyfam]

Re: Low Compression on one cylinder........

If the pluig looks "steam cleaned" then you have water intrusion in that cylinder. Whip the head off and check out the gasket.

 

[mrcj001]

Re: Low Compression on one cylinder........

If the pluig looks "steam cleaned" then you have water intrusion in that cylinder. Whip the head off and check out the gasket.

The plugs do look like they are really really clean.

There was quite abit of "vapor" coming from that cylinder....

Hopefully I`ll get time tomorrow to pull the head.....

Thanks for the input!

 

[ezeke]

Re: Low Compression on one cylinder........

In the meantime, keep that cylinder well lubricated; if it's getting water, the damage can be significant.

 

[BigB9000]

Re: Low Compression on one cylinder........

could a bad/stuck/corroded reed valve on that cylinder cause it to get less fuel than the others?
Leading to lack of oil, causing lower compression and increased heat, which leads to a leaking gasket, which leads to water intrusion.

 

[ezeke]

Re: Low Compression on one cylinder........

Compression reeding are not influenced by the reed valves, one way or the other under these conditions, though they can easily be the initial cause of the problem.

At 40 PSI you either have a blown head gasket or a bigger problem.

 

[mrcj001]

Re: Low Compression on one cylinder........

UPDATE:

What the hell happened:

UPPER #2 cylinder

I`m listening...............

There was a little bit of water in the cylinder

 

[whalerfreak22]

Re: Low Compression on one cylinder........

Yep time to rebuild or repower I myself would sell that motor for parts to buy a newer motor.

 

[trendsetter240]

Re: Low Compression on one cylinder........

Clearly some foreign metal object got in the cylinder. Most likely a piece of a broken ring.

That engine needs a rebuild, no way around it. The good news the folks on here can help you through it should you decide to take that route.

Cheers.

 

[emdsapmgr]

Re: Low Compression on one cylinder........

The top ring probably broke up. That's why you have the dents in the piston and head surfaces-from the ring pieces. Once they pound themselves between the cycling piston and the head, the small ring pieces blow out the exhaust ports. You may fine parts of the broken ring in the lower unit exhaust cavity behind the prop. Also why the compression is so low. The water diverters seem to be correctly in place, so an overheat on that one cyl is unlikely. Check for carbon around the piston skirts and around the rings. If carbon builds up enough it can restrict incoming fuel/oil from lubricating (cooling) the rings and they can break up due to lack of lubrication (overheating) on just one cyl.

 

[mrcj001]

Re: Low Compression on one cylinder........

I`m in the prossess of finding another engine...this one is too far gone for me.

There is a nasty gouge in the cylinder wall. I just dont think .40 over will do it.

Thanks for all your inputs.

 

[kenmyfam]

Re: Low Compression on one cylinder........

Looks like a ring let go.