How to test for dropped cylinder?

Discussion in 'Outboard Maintenance' started by Dillusion, Aug 5, 2013.

  1. Dillusion

    Dillusion devilray snob

    What I thought was a fuel issue previously is looking more like one of my cylinders is going out when the outboard heats up.

    Yamaha 50 2-stroke 2004

    From 0-3900 RPM the outboard runs just fine. From 4000-4900RPM the motor will for no reason while running drop to 4000 RPM and 10-20 seconds later jump back up to 4900RPM. When running over 4900RPM the motor does it, but less frequently, I think. WOT is 5300 RPM.

    The outboard has brand new fuel lines, brand new fuel filter assembly, brand new QD sockets, brand new fuel pump, and the carbs were just rebuilt. This leads me to believe something in the ECM/ECU/PowerPack/Stator is heating up at a certain temp and expanding, or one of the coils is going bad or also having trouble with the heat.

    How does one go about testing this at home? I could test it out on the water I guess, and bring a spark checker with me and somehow look backwards at the motor while running? lol Or I can bring my digital MM with me while out on the water and do some voltage measurements when the outboard gets hot from being at WOT- but I can't do any of this on the hose at home.

    Is there ANYTHING I can do at home to diagnose it while it's warmed up from idling?

    I am going to unscrew the bowl screws in my carbs tonight and bleed out whatever is in there and see if there is any sediment in the bowls warranting another carb clean...but I doubt it.
     
  2. rkmurphy

    rkmurphy Well-Known Member

    Go to Harbor Freight, buy yourself a $10-$15 compression tester, and report back. Hope it's nothing serious! I probably need to do this, as well...just for my own comfort.

    Have you badly overheated it? That Yammy 2 stroke shouldn't be giving you trouble like that!
     

  3. danville_marine

    danville_marine I Love microskiff.com!

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    use a timing light have some one watch while u run it see if light quits firing
     
  4. Dillusion

    Dillusion devilray snob

    Compression is fine, 90 on all cylinders with my cheap gauge.

    It hasn't overheated, it has an overheat alarm. If compression was bad it would be bad all the time, not intermittent. Though I could see hat your saying as the metal expands in the cylinders it could cause problems, but it's not a compression issue.
     
  5. rkmurphy

    rkmurphy Well-Known Member

    Is it just me or does 90 seem low? My knowledge is pretty limited here...
     
  6. Dillusion

    Dillusion devilray snob

    90 is 'low', but it's a cheap gauge and with a proper one it would probably register 110-120 like it should. When checking compression looking to see if all cylinders are within range is more important that what the actual # is...unless they are all saying 50psi lol than thats a problem.
     
  7. HaMm3r

    HaMm3r Well-Known Member

    Any chance it's ventilation of the prop that's causing the RPM fluctuations? Perhaps if you're running the prop close to the surface it's ventilating a bit, then all of a sudden it gets a little more bite and RPMs drop until it begins to slip again. That would explain why it does it less at WOT, since you'd be running higher on plane, thereby farther above the slip/grab water level.

    No idea how your motor is mounted, but if you can trim it down farther and test again, it might save you some headache.
     
  8. TwoKids

    TwoKids Well-Known Member

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    I like keelme's idea, I gotta remember that one, smart thinking. I have heard cut and creek say yammie electrical systems are rock solid more than once though. If it were mine just for the heck of it I'd complete a shock of ring free or seafoam then change the plugs just as a little preventative maintenance if nothing else and you never know you may get lucky and free something up. Many people swear by that stuff and Yamaha recommends it.

    Not to get off topic but just curious, what kind of oil are you running and do you premix or is it injected?
     
  9. Dillusion

    Dillusion devilray snob

    I already use yamaha ring-free every other tank of gas.

    I am not pre-mixing, I am using yamaha 2-M 2-stroke oil in the oil tank.
     
  10. Dillusion

    Dillusion devilray snob

    If it was slipping, my RPM's would go up- not down. I thought of that too and I am getting a cup put in my prop this week.
     
  11. TwoKids

    TwoKids Well-Known Member

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    I was thinking going with a serious shock treatment dosage, 2oz/gal If I remember correctly... assuming you have never done a super clean since you bought it. Standard every other tank PM type dosage is good to prevent buildup but if was already gunked prior to you buying it then the light dose prob wouldn't cut the performance robbing nasty stuff. Good luck to you with the troubleshooting, hope you can get it fixed, chasing ghosts is frustrating, been there...
     
  12. Dillusion

    Dillusion devilray snob

    Already did a shock treatment when I got the motor.
     
  13. TwoKids

    TwoKids Well-Known Member

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    Got some time and wear on those plugs yet where you can inspect for any deltas between them?
     
  14. DuckNut

    DuckNut Brandon, FL

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    From my knowledge this is somewhat accurate. Correct, that consistency is vital. But why would an expensive gauge read 30% higher?

    To get accurate numbers it is important to check when the engine is warm, not cold.

    Not gospel. Your compression can vary drastically between cold and warm as well as idle vs. speed and no load vs. load.

    Although it does not sound like a piston problem because the rpm's come back, can't rule it out. It does sound more like an electrical or intermittent fuel starvation.

    Since there only a few wires, can you test the resistance. Also- check the plug wires for resistance - not sure if yours are integral to the box or not.

    How did the fuel re-check go?
     
  15. Creek Runner

    Creek Runner Well-Known Member

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    Your issue if you are describing it right does in fact sound like an electrical issue. Does it do it every time you go out? You can use the timing light as pointed out above to determine if you have a spark issue keep a close eye on the light it may not just quit firing but start missing. You can not check the entire firing systems with out some special tools. But you can check some, I think I remember you saying you had a manual in it will be the procedures to check the components you can. Let me know if you need any help.


    Standing by
    Creek
     
  16. Creek Runner

    Creek Runner Well-Known Member

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    90psi is low, and I have used $15-$300 gauges and have never had a gauge read 25-30psi low unless it was broke. But I don't think it has anything to do with your current problem. It also may come up once you run it for 10 or so hours as I think I remember you saying the motor sat for a long period of time.

    Standing by
    Creek
     
  17. HaMm3r

    HaMm3r Well-Known Member

    Read what I wrote again...you're interpreting it exactly the opposite of what I was suggesting.
     
  18. Dillusion

    Dillusion devilray snob

    So you're saying it's the opposite...? I'm slipping all the time, and only 'biting' when the RPMs drop?
     
  19. Dillusion

    Dillusion devilray snob

    Yes it does it every time I go out.

    I do have the manual and will be reading it today in the electrical chapter.

    I wanna get this fixed so I can sell the damn boat with a clean bill of health ;D
     
  20. Creek Runner

    Creek Runner Well-Known Member

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    Let me know if you need any help, I might even be down in the lagoon area next week, might be able to meet up with you help diga.

    Standing by
     
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