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1987 Johnson 150 VRO fried/melted piston

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  • 1987 Johnson 150 VRO fried/melted piston

    After the shop called me and said that the motor was blown i brought it home and decided to look at it myself to see if it would be worth rebuilding. The Top right piston is melted but none of the others are. They actually look good along with the cylinder walls and the heads. No groves or build up. there is a broken wire going to the motor temp switch on the right side u can see in the pics. I do not know if it has been broke the whole time or if it was broke when they took the head off at the shop. Since it is the only piston that is messed up i dont know what it is. In my little brain since it is a cross flow motor then if it was fuel/air problem then it should show on more than one piston. And the VRO pump is off and I mix 50-1. wut could be the problem? I dont know so any help would be awesome. Thanks in advance.
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  • #2
    Re: 1987 Johnson 150 VRO fried/melted piston

    Looks like you have crud obstructing the cooling on that bank which likely caused the overheat. Having the overheat sensor wire broken prevented the overheat warning buzzer from sounding. A close inspection of that cylinders' carb and thermostat would also be a good plan.

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    • #3
      Re: 1987 Johnson 150 VRO fried/melted piston

      Unlikely it was incorrect fuel, since 5 of the 6 cyls like the fuel/mixture that you are using. You have a couple of possible problems: 1. I'd pull that carb apart that feeds the two upper cyls. Esp check the high speed jet that feeds that half of the carb that feeds that bad cyl. You may find some debris in the jet, clogging the fuel flow. Any fuel restriction will cause that cyl to run lean. ( and hotter.) Proper fuel flow keeps the cyls cool and lubricated. 2. From looking at the 4th picture, it looks like the water flow around that top cyl may be restricted. The water passage should look like the middle cyl, clear, open. Yours looks like a lot of crud has built up around the top cyl. A good cleaning is in order. Also, it's hard to see the rubber water diverter by the 4 o'clock position for the top cyl. 3. Looks like some debris may be restricting water flow around the middle cyl at the 7 o'clock position. Any water flow restriction can cause a single cyl to run too hot-and it may not set off a hot horn in the process. Is this engine used in saltwater?

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      • #4
        Re: 1987 Johnson 150 VRO fried/melted piston

        all of that bulid up around that piston/ cylinder i thought was part of the gasket set. but yes if I am going to rebuild then yes all of it will be getting cleaned real good. The carbs have just been rebuilt about two weeks before they said it was blown, only had a chance to take it out once and blew up then. and yes the motor has been used is salt water. Since I have had the boat everytime it goes is salt water if has been ran with fresh water when I get back to flush it out. I cannot speak for the owner who had it before me. Also the piston in pic 5 has writing on it and the only thing I understand is that it says that it has been bored .020 over but the pistons that I am used to looking at say OS. What does the KD and the other letters mean? There is a lot more play in the piston cylinder wall clerance then the max movement. I think that it is bored 30 over and to save money they just used over sized rings. has anyone heard of people doing this?

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        • #5
          Re: 1987 Johnson 150 VRO fried/melted piston

          Usually pistons are marked on the crown with the oversize information. #5 is an oversize. Hard to see any other oversize markings on the other pistons from the pictures. It is common (accepted) practice during an overhaul to just overbore one (or two) cylinders-those that are out of spec-leaving the remainder at standard bore. Likely any stock piston has stock rings, similarly-the .020 oversize piston will have a .020 oversize ringset. It's normal for these engine to have a lot of clearance between the piston and cyl walls when the engine is cool.

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          • #6
            Re: 1987 Johnson 150 VRO fried/melted piston

            ok well thank you for the clarification on the stamped numbers on the piston and all of the pistons are 20 over. and also thanks for the info of the piston clearance.

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            • #7
              Re: 1987 Johnson 150 VRO fried/melted piston

              I understand is that it says that it has been bored .020 over but the pistons that I am used to looking at say OS. What does the KD and the other letters mean?
              That a Wiseco piston.......yuck.
              When in doubt,Trim it out!!!
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              • #8
                Re: 1987 Johnson 150 VRO fried/melted piston

                ok well wut pistons should I opt for without spending an armand a leg? Wiseco pistons are decent pistons for a vehicle but dont know about boat motors.

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                • #9
                  Re: 1987 Johnson 150 VRO fried/melted piston

                  Pics of the heads are missing. If the head is damaged in the combustion chamber you will soon run into trouble again if just changing pistons. The reason for the break down might not be recently, but that it was not properly repaired at last re-build.
                  A damage as shown might be caused by multiple reasons, but with a common name:Lean burning.
                  It might have been a leaking top crank seal, defective cyl head, blocked carb jet, lack of fuel pressure and even faulty cooling like impeller and thermostats/pressure valves. You will need to go through it all.

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                  • #10
                    Re: 1987 Johnson 150 VRO fried/melted piston

                    wat wrong with Wiseco. Enlighten us please! Work great in all the auto engines that i put them in.

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                    • #11
                      Re: 1987 Johnson 150 VRO fried/melted piston

                      Nothing wrong with Wiseco provided you install them according to Wiseco specs (piston clearance). If reboring and using other brand than OEM, send the pistons (new) with the engine and ask to get thee cyl(s) bored according to clearance spec by manufacturer. If using OEM pistons, factory cyl specs may be used. Forged and casted pistons needs different piston clearance (cold) due to different expansion when heated up.

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                      • #12
                        Re: 1987 Johnson 150 VRO fried/melted piston

                        If that bad piston is a Wiseco- that may have contributed to the failure. Not a good idea to mix pistons from two different manufacturers in the same block. As Haffiman mentioned, pistons from different manufacturers will expand at different rates when the engine heats up to normal operating temps. Unless the machinist that overbored that cylinder took that into account the last time this block was worked on-could contribute to running problems and long-term durability.

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                        • #13
                          Re: 1987 Johnson 150 VRO fried/melted piston

                          the pistons are all the same make and the heads are good just like the cylinder walls and all the other pistons.

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                          • #14
                            Re: 1987 Johnson 150 VRO fried/melted piston

                            What did you find when you pulled the carb apart?

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                            • #15
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                              Re: 1987 Johnson 150 VRO fried/melted piston

                              i found nothing out of the ordinary. If there was any trash in there it must have passed through.

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