Low compression

Discussion in 'Outboard Maintenance' started by Sheremeta, Jul 13, 2012.

  1. Sheremeta

    Sheremeta Well-Known Member

    I did a compression test today on my 2000 Yamaha 50hp two stroke and the top and bottom cylinders read 120 but the center reads 80.

    The engine runs great and idles great. This engine has the best idle I have ever experienced as its low and quiet. No vibration or odd sounds. I can get up on plane fast and engine pushes the boat fine from top end speed to low end speeds. I just changed the plugs and ran the engine on the muffs for 20 minutes. The plugs on the top and bottom were clean but the middle plug was fouled and black looking already. I don’t know much about the engines history but I just completed taking the lower unit completely apart, replaced all rings on the gear case, changed shaft seals and everything looked good.

    Do I have a ring problem on the middle cylinder due to corrosion or carbon? I am going to do a ring free shock treatment this weekend to see if it helps before pulling the head. Then I will pull the head, clean the cylinders and change all the rings and gaskets. If I dont see any red flag damage what else should I do?
  2. Sheremeta

    Sheremeta Well-Known Member

    also where can I get a good torque wrench kit?

  3. fsae99

    fsae99 I Love microskiff.com!

    The old school field check for ring issues is to put an oz in that cyl and check compression. If compression goes up then most likely rings are bad, but could be rings not aligned properly, or cyl wall issue. All 3 require head removal.

    I've had good luck at harbor freight with torque wrenches. I check mine against my brothers snap on's and they have been right on. It is Harbor freight so YMMV.
  4. Sheremeta

    Sheremeta Well-Known Member


    I am in the process of decarbonizing the engine with a shock treatment of ring free and have been seeing improvements in my idle range. I am also 99% sure the head gasket is bad and plan to replace it this week. When doing that I will check the rings and cylinder walls.
  5. Creek Runner

    Creek Runner Well-Known Member

    Only way to tell whats going on is to pull the head, Sears would be my 1st choice for any tool if Snap on or Matco was out of the question.

    If the head gasket is bad then that could be the reason for low compression. 
  6. Sheremeta

    Sheremeta Well-Known Member

    Compared to all the other gaskets the head gasket looks the worst. It has the tell tale signs of failure and is greenish brown in areas and the factory paint on the cylinder head where it touches the gasket has light surface corrosion.

    I am pretty impressed with the ring free. I have been starting the engine and letting it idle high for 5-10min then shut it off for 30 minutes to an hour and come back and restart. Lots of smoke.

    I can do this without a shop manual but I would like to have one for reference. Does anyone know where I can find one?
  7. Sheremeta

    Sheremeta Well-Known Member

    Just sprayed deep creep into the carbs and stalled the engine to soak overnight. I made such a large cloud of smoke a neighbor called me.

  8. Sheremeta

    Sheremeta Well-Known Member

    well I removed the head cover today and found my problem. I have a chip in the piston head and deep scoring in the cylinder walls. I am going to get a price to bore and install a larger piston but will probably repower. I am going to look at new hondas this weekend.
  9. cutrunner

    cutrunner Cert. Yamaha technician

    Sorry to hear that
  10. Sheremeta

    Sheremeta Well-Known Member

    Thanks but its no big deal. I had plans to repower the boat anyways. The only upsetting thing is I cant go out and buy a new 2 stroke. How are honda outboards? I have never owned one.
  11. cutrunner

    cutrunner Cert. Yamaha technician

    Well everyone has their opinion...
    But in my professional opinion, comparing their hardware, wiring, metal to others..
    Theres cheaper ways to anchor your boat if you get my drift
  12. Sheremeta

    Sheremeta Well-Known Member

    Do you know if my engine can be resleeved or does it have to be bored smooth. I don't want to install a larger cylinder.
  13. cutrunner

    cutrunner Cert. Yamaha technician

    Any pics of the gouges in the cylinder wall?
  14. Sheremeta

    Sheremeta Well-Known Member


    It's pretty bad. Look at the marks on the head cover.

    Even if I take the head to a machine shop to bore the cylinder and reassemble new parts it's not worth it. A new piston, rings and head cover plus gaskets im putting too much into it. Plus there is that missing piece of piston that I will have to verify that is still not in the block. The head gasket was also toast and looks like water made it past it.
  15. dingdangdoo

    dingdangdoo I LOVE FISHING

    Did you use yamaha ring free and stabil or anything of that nature? I hope what my yammie is good to go, hopefully you get everything back to tip top shape.
  16. Creek Runner

    Creek Runner Well-Known Member

    I would take it to a good local yamaha dealer and have a complete overhaul done with a short block from Yamaha. Your looking at probably less than $3500 from the lower case to the carbs and everything in between. Cheaper than a new motor and will run for another 1000hrs.
  17. Sheremeta

    Sheremeta Well-Known Member

    Ya but for 3k more I can get a new F50.

    Engine was broken down and everything else is in good shape.

    Crankshaft and bearings


    Bad piston

    No pieces of metal were found in the engine and everything was pretty clean. It appears the damage was a result of the cylinder running lean. 3 new pistons were ordered from wiseco and the block should be back by Friday.
  18. cutrunner

    cutrunner Cert. Yamaha technician

    Get some chris carson reeds too!!
    And clean up the ports
  19. Sheremeta

    Sheremeta Well-Known Member

    Going to pass on the plastic reeds.

    The crank case and everything else will be smoothed out by hand with emery cloth as well as washed to remove deposits.
  20. Snookdaddy

    Snookdaddy Well-Known Member

    I feel you're doing the right thing. I had a rebuilt 90hp Johnson and a rebuilt Mercury 25hp awhile ago.. ran both for 4+ years and both ran like a top and had great compression when I sold them.. As clean as everything looks, I think you will be happy...

    As for the Yamaha 40-50hp outboards... They are supposed to be "bomb proof"... I have a 1997 40hp Yamaha on my Challenger and it has never let me down.. It only has about 350 hours, but it is a damn dependable motor that I have relied on for many trips to the Everglades.. I will be taking the same rig down this weekend and expect nothing less than butter-smooth performance..

    IMHO, I think that simple 2 stroke motors sometimes have an edge on more advanced tech.. I'd be f*&ked if a mother board or comp chip went out while fishing 25 miles deep in the Glades...

    What was your quote on the re-build? PM me if you do not want to disclose...


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