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  1. #1
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    Picked up a cheap omc v4.. why did she blow up?

    Picked up this 1994 johnson 90/65 jet. The two starboard cylinders were blown. Two port cylinders look great. I did notice the timing linkage connector was broken. Perhaps detonation? Thermostat issue? Dont think it overheated or leaned out, however.






  2. #2
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    I have a 84 OMC 140 cross flow that I have no idea how many hrs are on it. It caught a ring on # 3 cyl and hammered the piston and head. I was told that the performance V4s have a tendicy to do this with a lot of time on them. Your mota having 2 cyls with the same problem and a broken timing connection... you may have found the problem. I would check the carb bowls and jets also. Gary
    "12" Super Lite Tunnel (11') "88" 25 Yammy twin carb "BANANA SPLIT"
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    "Too much is never enough" Keith Richards " Dreams become reality via hard work and perseverance" G.A.Carbonneau

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  3. #3
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    That to me looks like over heat for sure, i have a 3 cylinder that has a piston just like that for damage.
    if you only have the omc temp sender that is on a buzzer they are useless.
    best to have a gauge for each bank so you know what is happening.

  4. #4
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    Not sure if this is relevant information but after further tear down the thermostat on the two blown cylinder side is stuck open

  5. #5
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    After sleeping all night and not being dead tired, i realized both pistons failed on the EXHAUST side, not the intake side. Same exact place. Click image for larger version. 

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  6. #6
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    I would sooner say over heated rather than too cold.
    were the little rubber diverters in the waterways in the right places ?

  7. #7
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    Quote Originally Posted by phillnjack View Post
    I would sooner say over heated rather than too cold.
    were the little rubber diverters in the waterways in the right places ?
    Believe so, all 4 in the same place

  8. #8
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    from what I can see the water passage looks to be very very clean, like a drop or two of boiling water has gone through it ?
    Hope you manage to get it re-built

  9. #9
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    Does not appear to be heat related. Check the ring locating pin. Since it is .030 over may not have had proper clearance.

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  11. #10
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    Quote Originally Posted by racer View Post
    Does not appear to be heat related. Check the ring locating pin. Since it is .030 over may not have had proper clearance.
    Are they wiseco pistons? someone could have put it together wrong. But most likely locator pin came out and let the ring spin and catch a port, possibly the top cylinder first and bottom cylinder got the rain.............
    "One of the penalties for refusing to participate in politics is that you end up being governed by your inferiors". Plato .

  12. #11
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    Quote Originally Posted by JoshKeller View Post
    After sleeping all night and not being dead tired, i realized both pistons failed on the EXHAUST side, not the intake side. Same exact place. Click image for larger version. 

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    Nah, everything flows towards the exhaust...
    "One of the penalties for refusing to participate in politics is that you end up being governed by your inferiors". Plato .

  13. #12
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    How do the reeds look? any broken? could have eaten a reed or two
    "One of the penalties for refusing to participate in politics is that you end up being governed by your inferiors". Plato .

  14. #13
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    This is what an overheat looks like

    Click image for larger version. 

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    "One of the penalties for refusing to participate in politics is that you end up being governed by your inferiors". Plato .

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  16. #14
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    Both Locator pins are present in each piston. I did notice the timing advance couple between the throttle and trigger base and completepy broken. Would that cause it?Click image for larger version. 

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    Click image for larger version. 

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    Click image for larger version. 

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  17. #15
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    I see the locator pins in the bottom rings, but what about the top rings that caught the port?
    "One of the penalties for refusing to participate in politics is that you end up being governed by your inferiors". Plato .

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