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Compression anylisis

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Remote 95

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Just compression tested johnson 115 that has issues, the following readings, 1/120, 2/120, 3/120 4/100. Motor runs but will not idle or stay running. When able to throttle up without going dead for short period, motor has definate skip as if not running on all four. Could anyone give me an idea of what I am dealing with?
 
Remove the cylinder head of the bank that has the low compression cylinder. A 20 psi difference indicates that inspection of that cylinder, piston, head, gasket, is required.

I assume that you do have proper spark on all four cylinders, however you do not mention that aspect. We need to know.
 
Did the problem begin suddenly? That compression would never cause a "constant" miss, or inability to run at idle. That #4 cyl. will still pull at 100 psi. The test raises some issues but will not produce the symptoms that you describe. Need a good spark jump test for starters. Do you know how to do one? I was just going to appeal to Mr. Reeves, you are in good hands now.
 
Model # should be posted.----Likely broken piston ring if a crossflow motor.-----Remove bypass cover to inspect piston rings on the 100 PSI cylinder.----Or remove cylinder head.----Lucky you if it is a blown head gasket.
 
Racer is on now, you have our top techs on the job, brother........that's not including me, I'm just the guy that blames ethanol on everything.
 
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Most Humble thanks to all. The outboard is a Johnson 115 ocean runner, Model # 0115SLEDR, the serial is G 03992055. I just purchased this boat with knowledge it had a problem with the motor. I have a call in to try to get in contact with the guy who had previously worked on it. Whomever worked on it has definately pulled the head on the #4 cylinder as you can see new head gasket material. If Im able to contact the guy who did that I hope he can tell me what he saw inside the block or whether he tried honing the cylinder or more if not then it may come down to pulling the head.
 
As far as a spark jump test I would be grateful for the info, I have done a firing test with a lawnmower and test light or screwdriover grounded to the block if this is what is meaned?
 
That is a looper motor and it does NOT use a head gasket , but uses O-rings.---Fewer problems with rings on those.-----I would say pull the head now.
 
Remote 95 said:
As far as a spark jump test I would be grateful for the info, I have done a firing test with a lawnmower and test light or screwdriover grounded to the block if this is what is meaned?
Click to expand...

A screwdriver shows you have spark, but it's not quite enough. You need to use an adjustable air-gap spark tester to correctly determine the health of your ignition. A good ignition system should be able to create spark that jumps a 7/16" gap.

Get a gap tester like this one from Amazon: https://www.amazon.com/Lisle-20700-...ocphy=9022539&hvtargid=pla-570785920751&psc=1
 
That is one sweet tester, brother.......my wife, Raquel, says I gotta have one......she'll trade me for a shopping spree at Victoria's Secret, .....we BOTH win!......
 
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