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Thread: Piston Damage

  1. #1
    seaobin's Avatar
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    Piston Damage

    Any ideas what could have caused this?

    This is the mag piston of a 750

    Good fuel
    Good Compression (before)
    New Bores and hone (.5 over)
    Might have been a little lean but we were adjusting that
    happened while at 3/4 throttle


    Who Knows
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  2. #2
    xplayer2885's Avatar
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    Did you have the hi-flow fuel pump at all???

  3. #3
    Moderator beerdart's Avatar
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    Oh Snap!!!
    It looks very lean!!! and or over heat. And when the pieces start melting at 5000 rpm is not good.. How does the cylinder look?

  4. #4
    seaobin's Avatar
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    yes to triple hi flow pump

    cylinder looks ok, matching surface scuff,

    Do youthink its possible that my engine shop didnt give me the correct bore?

    Definate scaring around the exhaust ports

    like it clipped the edges

    Sean

  5. #5
    seaobin's Avatar
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    How can I richen that cylinder up any more, My high screw has a tap on it that only allows for an adjustment of about 1/4 turn

  6. #6
    cut the tab off the cap! Z

  7. #7
    seaobin's Avatar
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    of course, why didnt I think of that

    Duh!

  8. #8
    Wankel till I die!! jonjonwells's Avatar
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    Those tabs are a fitting that goes over the brass adjuster. A pair of needle nose should be able to pull it off with no prob. Then reinstall when done.

  9. #9
    cheatin' piston popper addicted's Avatar
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    cut the part of the tab that sticks out toward the needle, then close the needle and install the cap with the remainder of the tab straigt up or down, whatever. That way you can tund them 360* and have the tab for reference on you adjustments so that you don't have to keep closing them

    were the carbs rebuilte when the rebuild was done? lines and filters checked? oil pump and lines checked?

  10. #10
    cameron da river rat's Avatar
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    looks to me like a lean sieze.
    did you champfer your ports after the bore?
    what was the piston to cylinder clearance?
    did you break it in properly?
    impropper/bad fuel would prob caused deto before it went and i dont see any signs of deto. looks like lean on fuel or lean on oil.
    front cylinder runs hotter since its next to the stator section, if your carbs have a little plastic cap with a notch on them, just pull it off with a pair of plyers and richen it up past what you had.
    remember after rebuilds, compression usually goes up compared to stock compression readings since you now have a little more cc's of air that is being compressed. hence, thats why the major rebuild companies redo the squish and widen the bore on the head a little to compensate.
    whats crappy about three cylinders is that it cost more to rebuild, and now you have to do all three again and go even bigger due to the damage.

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