whats wrong with running a motor with low compression in one cylinder?

[bob johnson]

I picked up a nice clean 1996 Johnson 115 hp Ocean Runner 2 years ago for a seamingly great price. the seller was up front and said it had low compression in one hole. but we ran the motor before I bought it. i got the controls ,m the oil tank , and the harness with the motor...as well as its stand...for $700.

the compression is 125 120 120 and 95.

I decarbed the motor and nothign changed...

my question is......what are the consequnces if i mount the motor on my boat and use it???

its a V4....60 degree looper.

i orginally picked it up for a spare parts motor.. because I have two others. But i have a starcraft Ill be refurbishing that could use a motor. this would be ideal...


thanks, hope to hear from you guys


bob

 

[trendsetter240]

Re: whats wrong with running a motor with low compression in one cylinder?

I'd first want to find out what caused the one piston to be so much lower than the rest. Possibly the carb on that cylinder is dirty and it was running lean causing the piston to score slightly. It also could have problems with the OMS pump.

Everyone always says "a failing VRO / OMS pump can't just damage one cylinder". That's BS because I've seen several motors that were run without oil and every single one failed on only one cylinder. Usually the hottest cylinder will fail first under lean conditions and the motor is usually stopped before the rest fail. Not saying this is what happened here but I would run pre-mix in the motor as a precaution.

As for running with one cyl 30psi low you could experience some vibration. Also, there will be a slightly uneven pressure of the crank bearings which over time may lead to failure. My personal opinion would be to tear it down and hopefully you can get away with just boring one hole, one piston and a set of rings. Pretty small investment for a nice 115hp motor.

Cheers

 

[F_R]

Re: whats wrong with running a motor with low compression in one cylinder?

You must find out why the compression is low. In most instances, continuing to run it that way will completely destroy that cylinder, if not more. Some possible resons for low compression:

1. Carbon/stuck rings. You already tried the magic juice method without success. May have to get more serious and physically remove it.

2. Broken rings. Will rapidly get much worse, destroying the piston and cylinder.

3. Broken piston. Worse than #2

4. Water getting into the cylinder. Washes out the lubrication, and causes extreme wear.

5. Running lean on that cylinder because of carburetor problems. Causes pre-ignition and broken pistons or rings or both. See #2 & 3.

 

[Chinewalker]

Re: whats wrong with running a motor with low compression in one cylinder?

Running it 'til it breaks often means the difference between a rebuildable motor and one that's nothing more than a parts motor...

 

[multimech]

Re: whats wrong with running a motor with low compression in one cylinder?

Because the piston is damaged as F_R said and I will add another scenario. The piston may have contacted the cylinder walls due to any number of reasons. Once they hit the wall of the cylinder like that, damage occurs and the damaged part of the aluminum will continue to stick in the hole and it will turn the engine into junk.

 

[boobie]

Re: whats wrong with running a motor with low compression in one cylinder?

Fix it before you run it or plan on spending big $$$$$ later.

 

[bob johnson]

Re: whats wrong with running a motor with low compression in one cylinder?

is it possible that the compression could be low because of a bad o-ring that surrounds the hole??? there is NO HEAD GASKET on this style motor. so if the head is slightly warped the metal to metal fit of head to block wont hold any compression....

I guess ill drop the head to check.. I was a long way from actually really thinking of installing the motor as is...

if the cylinder looks good..Ill replace the o-ring ,check the head for flatness, and go from there.

bob

 

[rockyrude]

Re: whats wrong with running a motor with low compression in one cylinder?

Add to all these scenarios, when it does fail you will be a mile offshore.