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  1. #1

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    GP1200R 66V broken piston pictures. Ideas to what happened?

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ID:	206879What we have here is a picture of a piston with about seven hours since rebuild. Any ideas to what went wrong?

    The cylinder was a SBT with steel liner, bored oversized for WSM piston.
    Engine was running good before failure and had normal compression.
    Oil line to #3 carb was connected, did not injest water, upper and lower connection rod feel normal, PV good, and ring alignment pins good.


  2. #2
    Wow... Definitely seems a little scary to do the rebuild myself now! What would cause this, so I know I don't make that mistake..

  3. #3
    That piston got stuck in that bore due to overheating. (There are many causes for overheating) If the oil line was still hooked up, then try spinning the oil pump to see if oil comes out of that particular tube. Sometimes a feed line can get pinched, clogged or otherwise fail to deliver the oil. Since the other cylinders seem to be fine, then I would look there. An over lean condition do to a failing carb diaphram could also cause one cylinder to be starved for fuel and again, heat up and become lodged in the bore. If the carbs have not had a rebuild in the last few years, then alcohol in the fuel just loves to distroy the diaphrams and they will fail at the worst times. If the cooling passages are good on the block and the head for that cylinder, then the cooling was not the cause, but check. You said it was running good prior to the blow up. Did it go while under wide open throttle? If it did, then the carbs are most likely the cause. If two out of three were functioning properly, then under load, it would burn up the one with no fuel. Something as simple as a piece of crud in a jet could also cause this. Two strokes are relatively simple but a catastrophic failure like you had is due to one of the three things mentioned. When or if you have a new block to install, do not cheap out and make sure to get or rebuild the carbs to make sure you are good. Be sure to have a new filter in line so that the fuel is perfect before it gets to the carbs. Some advocate premix and if that makes you sleep better, then ditch the oil pump and go that route but I have not personally experienced or know of anyone with an oil pump related failure on a Yamaha since I started working on them in 1969. (dirt bikes, outboards, road bikes, and finally watercraft.) Many have seen their motors blow up from oil delivery issues but the pump was not the cause, the lines feeding the cylinders were the fault. Finally, I do not know if the motor was built by SBT or it was a previous SBT build that you had punched out, but if whoever did the build miss sized the piston to the bore, then the failure could have been as simple as a piston that was too large for the bore. (it happens). Close tolerences on a two stroke are a detriment if the motor is pushed beyond its limit. My thought would be to have a reputable shop put together the parts, do the machine work and recondition your existing carbs. Good luck with the next one but have those carbs checked before you use them again. Just my two cents and probably worth a little less than that.

  4. #4
    agsbill's Avatar
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    Paragraph???

    Give us a break!

    Bill

  5. #5
    agsbill's Avatar
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    That piston got stuck in that bore due to overheating. (There are many causes for overheating) If the oil line was still hooked up, then try spinning the oil pump to see if oil comes out of that particular tube. Sometimes a feed line can get pinched, clogged or otherwise fail to deliver the oil. Since the other cylinders seem to be fine, then I would look there.

    An over lean condition due to a failing carb diaphram could also cause one cylinder to be starved for fuel and again, heat up and become lodged in the bore. If the carbs have not had a rebuild in the last few years, then alcohol in the fuel just loves to distroy the diaphrams and they will fail at the worst times.

    If the cooling passages are good on the block and the head for that cylinder, then the cooling was not the cause, but check. You said it was running good prior to the blow up. Did it go while under wide open throttle? If it did, then the carbs are most likely the cause. If two out of three were functioning properly, then under load, it would burn up the one with no fuel.

    Something as simple as a piece of crud in a jet could also cause this. Two strokes are relatively simple but a catastrophic failure like you had is due to one of the three things mentioned.

    When or if you have a new block to install, do not cheap out and make sure to get or rebuild the carbs to make sure you are good. Be sure to have a new filter in line so that the fuel is perfect before it gets to the carbs. Some advocate premix and if that makes you sleep better, then ditch the oil pump and go that route but I have not personally experienced or know of anyone with an oil pump related failure on a Yamaha since I started working on them in 1969. (dirt bikes, outboards, road bikes, and finally watercraft.) Many have seen their motors blow up from oil delivery issues but the pump was not the cause, the lines feeding the cylinders were the fault.

    Finally, I do not know if the motor was built by SBT or it was a previous SBT build that you had punched out, but if whoever did the build miss sized the piston to the bore, then the failure could have been as simple as a piston that was too large for the bore. (it happens). Close tolerences on a two stroke are a detriment if the motor is pushed beyond its limit. My thought would be to have a reputable shop put together the parts, do the machine work and recondition your existing carbs. Good luck with the next one but have those carbs checked before you use them again. Just my two cents and probably worth a little less than that.

  6. #6
    Yamaha Mechanic TunedFX140's Avatar
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    we have dealt with SBT before we got a engine one time from them that ran for 5 mins, literally, upon re dissassembly we discovered that one of the main bearings that supported the crank didnt have all of its balls in it but yet the races were just fine the bearing just didn't have all the parts it needed. It destroyed the engine though since then we will not order a SBT engine to save our souls. If you came in asking for us to put in a SBT engine we are gonna tell you there is a shop to the east and to the west.

    Now we just re build stuff if it needs a crank we order from Hot rods, cylinders their non borable so we send them off to Langcourt to be re coated, carbs are done in house, heads can usually be re cleaned unless their really bad then its just gotta be replaced.

    I have seen one oil pump fail and thats because there was water in the oil tank it ran for a bit came apart then sat for a while then the oil pump rusted up and was no longer pumping the little cam that the cable goes in was stuck at wide open position

  7. #7
    agsbill's Avatar
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    Thanks ROCK STAR!!

    er....I should say

    TunedFX140

    Welcome to GreenHulk!!

    Bill

  8. #8
    Bill and all members,

    My apologies.

    Missed the professional writing course required of forum membership, however, with your expert help and guidance, will in the future separate subject matter by adding the necessary open space. (paragraphs)

    As the original thread starter indicated, he was looking for ideas on what happened.

    I trust that the "opinions" offered were useful to someone even without the spacing.

    If he had requested a properly formatted and proofed term paper, I'm sure there is an appropriate forum for that, too.

    Again, thanks for the tips.

    cnyman

  9. #9
    agsbill's Avatar
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    Awesome,

    Paragraphs make info so much easier to read.

    You have a good knowledge of these skis, so your knowledge is certainly welcome here!

    Thanks..

    Bill

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