Compression check results

Discussion in 'Outboard Maintenance' started by cslascro, Nov 7, 2012.

  1. cslascro

    cslascro I Love microskiff.com!

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    Hey gurus!

    My 2005 30 hp Johnson 2-stroke with electric start was making a strange grinding/clanging sound at high RPM or in the mid-range under load (plowing or turning while trying to get on plane).

    I did a compression test today and got an equal 90 psi in both cylinders. I was expecting to get closer to 100, but they're both equal and it doesn't seem terribly low.

    My owner's manual says nothing about cylinder compression.

    How do these numbers sound?

    BTW, when I pulled the cowling cover off I noticed that the starter pinion was engaged. It is showing a small amount of rust in the gear teeth. If this is not disengaging after the motor starts, I'm thinking it could be the culprit.
     
  2. cslascro

    cslascro I Love microskiff.com!

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    If I have a blown head gasket, is it a terribly involved process to replace it? I'm assuming that there aren't any touchy things to deal with on a simple little 2-stroke like this since there are no rocker boxes and such.
     

  3. Dillusion

    Dillusion devilray snob

    Did you just use a cheap $20 compression tester? Those tend to not be too accurate as far as their readings go, but if both cylinders are the same you should be OK. This is what I've read doing research though, I'm not an expert.

    I have a cheap compression tester I got on Amazon for like $14...on my 2002 mercury I get 110PSI on both cylinders with no % difference between the two.

    I'm 'supposed' to be getting around 120-125PSI, but I attribute that to the gauge being shitty and cheap. As long as they match up it seems that they should be fine.

    I could be wrong, though ;)
     
  4. cutrunner

    cutrunner Cert. Yamaha technician

    ^ your right
    Did an experiment inthe shop one day, as a bet.
    One of the guys that just bought a $350 snapon tester was acting like an azz so we all compression tested the same motor, with out different guages.
    There was a 20lb difference from the lowest to highest guage.
    But they were all equal, and thats what matters.
    If your ever unsure , always try a second tester before taking the motor apart :cool:
     
  5. cutrunner

    cutrunner Cert. Yamaha technician

    As far as the op's question goes:
    On some motors the starter armature is supposed to stick up in the flywheel teeth but go down once started.
    Go outside and start the boat up with the hood off, if the armature goes down once its running your good to go. If it stays locked in the teeth, you gots problems
     
  6. fsae99

    fsae99 I Love microskiff.com!

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    Was that CC done with throttle open or closed? My Johnson manual say to preform CC with throttle WFO. Just for grins, I did CC on mine a while back and there was 25PSI difference between throttle WFO and closed.
     
  7. Creek Runner

    Creek Runner Well-Known Member

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    #1 I'm glad you did a compression check 1st!!!

    #2 90lbs is low for that engine, that's a pretty high compression motor 110+ seen some as hish as 130.

    # I don't think that is your problem though, 90lbs on both cyc. (assuming that's what it is, maybe higher if you were to use a better gauage if you used a cheap $20 guage) Nor do I think you have a blown head gasket, it doesn't cause a grinding noise. But to answer your question it's pretty straight forward to change on your engine.

    How does the motor sound at idle? How does it sound on the hose? Pull the cowling and rev it up in netural by the carb? Pull the oil drain plug in the LU and see if you have metal frags.

    That motor should not have the starter engaged when not running, Put some grease on the bindex shaft and work it up and down by hand,

    Geeze that sounds kind of ----- :D but you get the point
     
  8. Creek Runner

    Creek Runner Well-Known Member

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    Is that your version of WOT, wide f------ open (WFO)! LMAO :D Never seen WFO before but I like it!

    That engine should not be done with the butterfly open.
     
  9. cutrunner

    cutrunner Cert. Yamaha technician

    Never heard of WFO?
    What rock u been livin under creek? ;D
     
  10. Creek Runner

    Creek Runner Well-Known Member

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    Yeah I know right, I have heard it but never seen WFO or actually used for a mechanical term. Just usually a drunk poker run guy or at the Sand bar. LMAO :D
     
  11. cslascro

    cslascro I Love microskiff.com!

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    Thanks, Creek Runner.

    I'll pull the lower unit drain plug and check there.

    The motor sounds absolutely fine at idle (in gear or not). I'm usually pretty sensitive to an off-sounding motor having grown up around two-stroke rental four-wheelers. That's what's been puzzling me. It seems that if it were internal I'd be hearing something at the low RPMs, or it would at least run rough.

    The last time I had the boat out I had my wife take the wheel while it was making the grinding noise so that I could listen closely and try to determine the source. I should have pulled the cowling at that time.

    It really didn't sound like it was coming from the lower unit, but I'll definitely check and see. Again, I would think it would at least rattle or shift hard at idle if it were a lower unit problem.

    I'll keep you all updated. Thanks for the great responses!
     
  12. Creek Runner

    Creek Runner Well-Known Member

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    Might be the top or bottom bearing, under load once it gets hot? How do the plugs look?
     
  13. fsae99

    fsae99 I Love microskiff.com!

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    Ha Ha, I thought the OP might not know what WOT was but would surely know WFO.

    "Just usually a drunk poker run guy or at the Sand bar. " I have resembled that remark in the past.
     
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