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1984 Johnson 115hp melted piston head (pics)

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  • 1984 Johnson 115hp melted piston head (pics)

    1984 Johnson 115hp
    Okay so I just had this motor serviced rebuilt carbs, new spark plugs and wires, new impeller rebuilt water pump, compression was good and new gear oil. today after running hour and half some mins after all this work running at 5600rpm 25-28 water pressure the motor droped from 5600rpm to 2700rpm WOT and died, motor cranked back up but would only do 2700rpm-4100rpm with a strange knock so it was less than a mile from my house dock to get it home maybe 7 mins back at that rpm. you will see what the knock was, i can rebuild this motor i have rebuilt a car motor so with the pics and as far as i have gotten into it i would like to have any info on what anyone thinks, that would have leaned only one piston out and not the other one, being i have never worked on a 2cycle boat motor is there anything special i need to look at as the source of this injury. I have never heard of the top of a piston melting with such little damage and what would cause that, here are all the pics i have. also that is the only piston with damage i check the other three.







  • #2
    Re: 1984 Johnson 115hp melted piston head (pics)

    Let me guess:
    NGK spark plugs ??
    Please no PMs. My mailbox is full anyway......

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    • #3
      Re: 1984 Johnson 115hp melted piston head (pics)

      yes, NGK R BZ7HS 10

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      • #4
        Re: 1984 Johnson 115hp melted piston head (pics)

        Originally posted by tx1961whaler View Post
        Let me guess:
        NGK spark plugs ??
        DONT leave us like that!!!!!!!!!!!!!!!!!!!!!

        I have been trying to learn all that i need to prevent this type of stuff...I am really dying to know if a SPARK PLUG ALONE can do this??

        why are the other cylinders not also damaged if it is due to the plugs??

        and why no over heat alarm????

        bob

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        • #5
          Re: 1984 Johnson 115hp melted piston head (pics)

          Thats pre-ignition from hot spot in cylinder,defective pack or bad coil. I have been using NGK's in all my customers engines for years(except 60 degree's) and never had piston problems. I just attended a CDI Electronics ignition class and the OMC coils are a very weak link due to their design. They use a powdered metal core and it is subject to breakage internally from being dropped, washers not installed on mounting bolts and over-torquing mounting bolts. This causes the coil to have 2 magnetic feilds of force which can cause a impulse spike into the pack at discharge damaging it, cause it(coil) to double fire or a intermittent miss. The only way this can be tested is with a induction coil tester and CDI can test them for you. As for plugs, heat range numbers used by manufacturers are not universal, a 10 heat range in Champion is not the same as a 10 heat range in NGK. Some numbering systems are opposite the other,ie: (Champion, Autolite, Splitfire), the higher the number, the hotter the plug. For Japanese manufacturers (NGK, Denso), the higher the number, the colder the plug. The worst thing that can happen from too cold a plug is a fouled spark plug. I am going to lean more towards the spark plug repair insert, Inserts for aluminum heads are made of aluminum and are hard anodized to prevent them from ever stripping again. It is very important to understand that all coil inserts made of stainless or steel simply transfers heat to slowly and will not transfer heat fast enough to allow the plug to cool and operate at the prescribed heat range. If a steel insert is used in an aluminum head it will fail to maintain metal to metal contact with the head as the engine heats up. Aluminum expands at a rate of 2.8 times more than steel. The aluminum will simply expand away from the insert and the plug will begin to loose its ability to transfer heat. If the spark plug overheats, pre-ignition and detonation can occur.
          Just my $.02
          When in doubt,Trim it out!!!
          Business Owner & Certifed Technician
          Boats I own & use
          20ft Javlin w/250 Junkrude
          17ft Bullet w/200 Merc (SOLD!!)
          13ft Allison w/150 I-6
          17ft Hydra-Sport 175GT
          15ft Allison w/75 Stinger(project)
          14 x 48 Drifter with 1100SS Merc

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          • #6
            Re: 1984 Johnson 115hp melted piston head (pics)

            I've seen similar damage caused by running lean. It's more common with jet skis being run with low fuel supplies, but worth checking for air leaks or blocked jets.

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            • #7
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              Re: 1984 Johnson 115hp melted piston head (pics)

              Had a guy tell me that a exhaust port pluged with something can casue this to happen.

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