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Thread: Yamaha 50hp Compression

  1. #1
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    Default Yamaha 50hp Compression

    I was checking compression on a jet ski I took possession of and didn’t believe the readings I was getting on my cheap Chinese compression checker. So I put it on my 50 Yamaha because I just new it had good compression as it starts and runs great, low and behold it reads 60psi on all 3 cylinders. So now I just know the cheap ass gage is the problem, and get a certified gage out of my work truck and get 62 psi on all three cylinders again. Plugs out, throttle wide open, but it was a cold engine. Am I screwed or about to be with this motor? Like I said it runs great. It’s a two stroke.
    Last edited by scmoose; 01-08-2020 at 05:11 PM.

  2. #2
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    That sounds low. Try it with the engine warmed up. If still low, de carb it (you should do this yearly) and try it again.

  3. #3
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    Yamaha stock compression should be between 115 and 120
    Hancock 70 Yamaha

  4. #4
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    Might need to have the certified gauge re-certified

  5. #5
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    Quote Originally Posted by Gone South View Post
    Might need to have the certified gauge re-certified
    Ah it’s good

  6. #6
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    should be on a warm engine with the throttle plate wide open.
    That does sound low but they were all +/- even, so that is good.

  7. #7
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    To find the good news at least all cylinders are close together.

  8. #8
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    Just seems weird that it runs as good as it does but shows compression so low on all three cylinders. I’ll warm it up tomorrow and take another reading.

  9. #9
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    I have seen another 10lbs psi or more from cold to warm.
    A fully charged battery is needed and spin it til the gauge stops reading.
    Last edited by ecu1984; 01-08-2020 at 07:19 PM.

  10. #10
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    I would doubt all three would be exactly 50% low on compression....

    3 hole Yamahas should be 110-120 depending on condition. If in doubt after checking with another gauge, look into performing a leak down test on the cylinders
    Last edited by FishSticker; 01-08-2020 at 07:13 PM.

  11. #11
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    Ran it again on a warm engine, idled on the muffs 5-10 min and no change, all within 2lbs, around 62 psi. So now what, run it until it pops, start looking into a rebuild, sell it and buy a 4 stroke?

  12. #12
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    Not to be undermining your knowledge, but you are holding the key until the gauge stops moving correct? Might make 4-5 cycles before it does.

  13. #13
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    Yeah holding it until the gage maxes out while holding throttle open.

  14. #14
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    Yeah holding it until the gage maxes out while holding throttle open.

  15. #15
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    I’m not suggesting this is going to work miracles, but have you ever de carboned your engine? I’ve seen it make a pretty substantial improvement on compression.

  16. #16
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    Have not but I’ll look into it

  17. #17
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    It’s just not common for all your cylinders to lose equal compression all at once. That thing wouldn’t be running well it it only makes half of its original compression

  18. #18
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    Quote Originally Posted by Gone South View Post
    It’s just not common for all your cylinders to lose equal compression all at once. That thing wouldn’t be running well it it only makes half of its original compression
    I’m with you, I’m baffled as well. Two different compression tools. I may try to borrow one from the auto parts store and see what it said.

  19. #19
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    De carbing is a good idea but I can’t see it accounting for that much of a drop.
    As said above do a leak down test

  20. #20
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    Checked throttle linkages to ensure all carbs are fully opening?

    Leakdown definitely.
    Last edited by SCmudder7071; 01-10-2020 at 01:17 PM.

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