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Yamaha 5C starts then stalls after a few seconds

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  • Yamaha 5C starts then stalls after a few seconds

    So I bought a used Yamaha 5C earlier this year and have been having a lot of trouble getting it running well since. First of all, a bit of background - the guy I bought it off said the head had been replaced by the previous owner, but he hadn't used it as it came with a boat that he already had a motor for.

    The outboard will eventually start after a lot of pulling, however it runs a bit rough and for only a few seconds if you give it a little throttle before abruptly stopping.

    I started with the usual spark and compression tests which were all up to scratch. Thus I assumed that it was a carb issue, so I ended up just replacing the entire carb with a brand new one as it wasn't expensive, plus a new spark plug just to play it safe. However the issues still persisted!

    Next I noticed that the compression was a bit higher with oil introduced to the cylinder, so I decided to replace the piston rings and thought I might as well replace the piston while I was at it... Lucky I did as the piston had a small gash in the side of it. But that got me thinking, how did that happen? So I then decided to completely tear it down, replace all the gaskets and seals, and put it all back together with gasket maker where necessary as per the service manual. Meanwhile, after clearing a lot of gunk built up behind the cylinder head cover where the coolant water flows through, I noticed that the cylinder/head had clearly been damaged and welded back together (note that the head and cylinder are a single part for this model, thus no head gasket)... That explains the damaged piston. From the inside of the cylinder no damage was visible, so perhaps the head had cracked due to overheating? Anyway the compression is good now and the same with and without oil introduced to the cylinder (95PSI, good for a 25-30 year old engine), so I am struggling to believe that that is part of the problem.

    IM BAFFLED! If anyone could provide any further advice that would be extremely appreciated.

    In summary, I have replaced all of the following parts and the issues still persist:
    • All gaskets and seals
    • Carburetor
    • Piston and rings
    • Impeller
    • Thermostat
    Other things to note - the reed valves are in good condition, water flow is good.

    Many thanks in advance!

  • #2
    95 is on the low side. Doesn't matter how new or old the engine is, it needs enough compression to run.

    How did the cylinder walls look, no scarring even thou the piston was?

    With the new rings, ring end gap was within tolerances (they were checked, yes)?

    "Gunk" as in salt water corrosion or ?

    Next, I would do a leak down test, (piston at TDC) and see how "tight" the engine is..


    1997 Angler 204, Center Console powered by a 2006 Yamaha F150TXR


    • #3
      Hi Townsends, thanks for the advice.

      The cylinder walls look good, no scoring.

      I haven't checked the ring end cap but I will do thanks.

      The gunk was whiteish in colour so I'm assuming it was probably a build up of salt and calcium deposits.

      I'm trying to source a leakdown kit, however I'm starting to think that the weld in the head might have corroded through. Water sprays out the telltale erratically as if it’s being mixed with gasses, rather than in a nice uniform stream. And the weld is between the combustion chamber and the water coolant chamber. I guess the leakdown test will confirm this too.



      • #4
        Sounds like you found the problem..

        With the "fixed" crack, once hooked up to a leak down test, soapy water sprayed in the area will bubble up as well...

        Sucking in water in the combustion chamber from the corroded head would also make running about impossible..

        1997 Angler 204, Center Console powered by a 2006 Yamaha F150TXR


        • #5
          Performed a (poor man's) leak down test and there's definitely a large hole in the head! Surprised it even ran at all.

          Have ordered a replacement.


          • #6
            That'd do it!

            Plz post back once up and running.

            1997 Angler 204, Center Console powered by a 2006 Yamaha F150TXR


            • #7
              Replaced the crankcase and it’s running like a dream, first pull!



              • #8
                dang it man that looks brand new,
                how much did it cost and where did you find that crankcase


                • #9
                  Found a brand new aftermarket one for $130 here:

                  Have bought parts from them in the past so I trusted the quality would suffice.

                  Shipping only took 2 weeks from China, can't complain!
                  Last edited by nickpitcher; 1 week ago.