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  1. #1

    Compression loss on a new engine

    I got a 700cc Polaris twin professionally rebuilt last summer. It had 145psi in both cylinders at the end of break-in. The engine still runs well and I get the same top water speed, but the compression in one of the cylinders declined to 130psi. Should I be worried? Is there some trick that can fight this trend? Adding a few drops of oil to the rear cylinder brings its compression to around 140.
    I have only 20 hours on the rebuilt engine. The sparkplugs look good, I've been premixing mineral 2-cycle oil and I am pretty sure that the engine never overheated.
    I'll appreciate any advise on this!


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    What was the maximum sustained RPM on the water last year after break-in?

    What is the RPM now?

    RPM tells the story regarding engine health, not indicated speed on the display.

  3. #3
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Are you using the same compression tester as last year?

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    bowsniper's Avatar
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    Check the cylinder heads bolts and base nuts for proper torque. if any are loose, you will loose some psi. It happened on my slt. are all the base gaskets and head gaskets all the same? are all the heads the original heads for this motor? or was 1 of them changed from another engine? I replaced 1 on mine that was stock and got more psi than the other original one.. no idea why though. maybe differences from motor to motor.

  5. #5
    blegan's Avatar
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    Quote Originally Posted by bowsniper View Post
    Check the cylinder heads bolts and base nuts for proper torque. if any are loose, you will loose some psi. It happened on my slt. are all the base gaskets and head gaskets all the same? are all the heads the original heads for this motor? or was 1 of them changed from another engine? I replaced 1 on mine that was stock and got more psi than the other original one.. no idea why though. maybe differences from motor to motor.
    I was doing a little research about break in period & was surprised how methodical this part of the process is! But one of the important parts was that you re tourqe the jugs & heads on these fuji motors after the break in preiod. Thats where I would start also.

  6. #6
    I am pretty sure both the RPMs and the water speed are the same now as at the end of break-in. I get close to 7000 RPMs and 55mph (unless the water is too choppy). The engine really sounds and feels Ok. I use the same compressometer that I had for years, and tried taking compression after running the jetski on two different days with the same results. I did have one issue recently that I forgot to mention. A friend took my jetski for a ride and did not put the lanyard on his wrist, so when the jetski flipped the engine did not shut down, and also unfortunately he did not kill it until some water got in and stopped the engine. This happened on idle RPMs so I was hoping that it should not have killed the engine. I did see some water in the carburetor after that, removed the sparkplugs and let the engine dry out a few hours. Do you think this could cause a permanent loss of compression? And what are my options at this point?

  7. #7
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by Dawg View Post
    ...friend took my jetski for a ride and did not put the lanyard on his wrist, so when the jetski flipped the engine did not shut down, and also unfortunately he did not kill it until some water got in and stopped the engine.

    This happened on idle RPMs so I was hoping that it should not have killed the engine.

    I did see some water in the carburetor after that, removed the sparkplugs and let the engine dry out a few hours.

    Do you think this could cause a permanent loss of compression? And what are my options at this point?
    Water ingestion can certainly crack pistons and do other damage, such as bent connecting rods, even at idle RPM. It can happen just cranking the engine with water inside.

    Did you run the engine right after getting the water out of the spark plug holes?

    Did you ride it right after getting the water out?

    Without a thorough investigation, we would just be guessing what has, or hasn't happened inside that engine.

  8. #8
    Bernie's Avatar
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    letting it dry out is not what you do .You need to run it for a couple of hrs as oon as you can get it going or you will be rebuilding it .This burns all the moisture out and you then fog it .

    Bernie

  9. #9
    When they fished out the jetski from the lake only a few things happened:
    1. I removed the flame arrestor from the carb and noticed a few drops of water.
    2. I removed the spark plugs and one of them seemed to have a bit of water.
    3. I let it sit on the sun a few hours
    4. I pushed the start button briefly to see if the engine turns. It seemed to turn easily and did not make any weird sounds.
    5. I cranked it for several seconds and watched some drops of water coming out of the sparkplug hole.
    6. Then I put the sparkplugs in, started the engine and let it run for 20 seconds or so. At first it was only running on one cylinder, then the second cylinder kicked in.
    7. Then I came home and took compression - the cylinder that had water in it was at 130, and getting to 140 if I add oil. I don't see any broken gaskets or leaks.
    8. Since then I took the jetski to the lake a couple of times, added 3 hours, and it seems to be working as well as ever before (including WOT, acceleragion, idle, etc.) I took compression after every ride and it always shows 10-15psi difference between the two cylinders. I am no longer sure if I am correctly linking this compression issue with the water getting into the carburetor... But I definitely had 145psi in each cylinder after break-in. I wonder if this issue could have developed gradually? Could it be that I am not premixing oil the right way? I usually just add it to the full tank and hope that it gets mixed up on the way to the lake which is 10 miles away... Not sure if there is a better way to stur the oil with gas?

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