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  1. #1
    SMLaker's Avatar
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    What causes a lot of GP1200R rear cylinders to burn

    I will be installing an SBT rebuilt 1200PV engine in my Y2K GP1200R and from what I have seen, there seems to be a common failure of the rear
    piston/cylinder burning up. I have a theory the serial fuel rail delivery method is to blame for causing a lean condition on the last carb in the
    serial line. I will attempt a parallel fuel feed setup to see if that distributes fuel more evenly across the three carbs.

    I think the condition of my plugs speaks volumes. First carb to receive fuel, plugs has carbon buildup and has a nice oil ring (kind of rich).
    Second carb to receive fuel, plug looks almost perfect, last carb to receive fuel, plugs is white and showing very lean condition.

    I have already ruled out a dirty carb, hole or leak in the case, bad shaft seals.

    I am wondering if ANY OTHER design or mechanical problem may be causing this over-abundance of rear cylinder failures. Like.....

    Water circulation in the head/cylinder is poor. If so, what mods are available?
    Reed not functioning correctly. etc...

    (Again, I am almost positive it is the serial fuel delivery setup. First carb gets too much fuel, second gets just enough, third carb doesn't get
    enough fuel at WOT).
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  2. #2
    SMLaker's Avatar
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    OOps, also. when I do the dual feed parallel fuel delivery mod (install T's between carbs, split main fuel line with a Y, cap off OEM fuel input port. I do not plan to
    use the reserve fuel line as the secondary feed (as shown in other posts). I don't think I need MORE fuel, just think it needs to be more evenly distributed. Do you
    see any problem with this?

  3. #3
    Julian's Avatar
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    I do'nt really see the benefit of this setup, since the fuel pressure is is build up by each carb, and the restictor keeps the pressure inside.

    You could better try to bore out your restricors, and place a restrictor ( main jet ) in your return line.
    This will even out the pressure over the carbs, and even the 3 carbs work together to build up pressure, and even will work as a backup for eachother.

    There is lot of info on this to find.

    If thats done, its all up to finetuning, which has to be done correctly to get the right mix in each cylinder.

  4. #4
    SMLaker's Avatar
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    Quote Originally Posted by Julian View Post
    I do'nt really see the benefit of this setup, since the fuel pressure is is build up by each carb, and the restictor keeps the pressure inside.

    You could better try to bore out your restricors, and place a restrictor ( main jet ) in your return line.
    This will even out the pressure over the carbs, and even the 3 carbs work together to build up pressure, and even will work as a backup for eachother.

    There is lot of info on this to find.

    If thats done, its all up to finetuning, which has to be done correctly to get the right mix in each cylinder.
    Thanks J. I have looked at the triple and dual parallel feed setups, boring out the restrictors and using a jet in the return line to get the right
    pressure. Totally Cool! I just can't seem to find an explanation for what my plugs are telling me....except for the serial fuel feed theory. I
    understand what you said about them working together but.....what else could cause this condition? The progression from rich to lean is
    so obvious. Arggggh!

    I also think Yamaha's cooling water pickup design sucks. A single inlet on the side of the impeller case that is "out of the water" when up on plane.

  5. #5
    Julian's Avatar
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    If the pressure is right on each 3 carbs, doesnt mean the fuelmixture delivery is the same in each cylinder:
    You probably just need to adjust your high's and low's correct for each carb. this sound easy, but is a precision-job.
    There's a lot to find, just search,

    The coolingwater pickup is fine, there is plenty of waterpressure on that side of the impeller, and certainly when in plane, trust me.

    " Water circulation in the head/cylinder is poor. If so, what mods are available?" is it poor in your setup? what do you mean

    "Reed not functioning correctly. etc..." What do you mean?

  6. #6

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    Were you premix or injection? If injection, were the oil lines still connected to carbs and flowing? What was the matching compression numbers with those plugs? Was it a quick burn/failure or over time? The most common and majority I've read about is simply oil injection failure (oil line slips off carb due to poor mfr decision/design to secure them with plastic zip ties)...or an intake/exhaust modification was made that created a lean condition & without the appropriate carb adjustment/rejetting. I had #1 oil inj line slip off, piston and rings burned up on my 66v engine. After close inspection of the aftermath, I chose to rebuild versus SBT replacement. My crank looks good and already had cylinder replated by Millenium (other 2 cyl honed)...I hope to do several upgrade mods (intake/FA upgrade, d-plate, carb rebuild/rejetting, primer kit, premix switchover) and save $600+ vs. SBT (I also wanted the challenge/experience)

  7. #7
    SMLaker's Avatar
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    Quote Originally Posted by Clarence View Post
    Were you premix or injection? If injection, were the oil lines still connected to carbs and flowing? What was the matching compression numbers with those plugs? Was it a quick burn/failure or over time? The most common and majority I've read about is simply oil injection failure (oil line slips off carb due to poor mfr decision/design to secure them with plastic zip ties)...or an intake/exhaust modification was made that created a lean condition & without the appropriate carb adjustment/rejetting. I had #1 oil inj line slip off, piston and rings burned up on my 66v engine. After close inspection of the aftermath, I chose to rebuild versus SBT replacement. My crank looks good and already had cylinder replated by Millenium (other 2 cyl honed)...I hope to do several upgrade mods (intake/FA upgrade, d-plate, carb rebuild/rejetting, primer kit, premix switchover) and save $600+ vs. SBT (I also wanted the challenge/experience)
    The last time #3 burned up (4 years ago) I changed to a premix 40:1 and eliminated the oil pump. I am running the RIVA D-Plate, Chip and
    free-flow exhaust. Stock intake. No leaks in the intake or exhaust manifolds. I had planned on a rebuild of this engine but the piston shattered
    and sent shards of aluminum throughout and punched a hole in the bottom of the case. Found pieces of piston in all three exhaust ports on the
    manifold. The reason I posted the questions is: the local PWC shop said they see this failure all the time with this make/model. Always the
    rear cylinder. If you look on YouTube there is also a video of the exact same failure. Everywhere I go, it's always the rear cylinder. Might need
    to put a bigger high speed jet in there. The reeds are in good shape but I may spend the extra $100 for a new set, or go with CarbonTech HT
    reeds for $172 a set.


  8. #8

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    The only problem with your theory is the #3 carb is the FIRST carb to get fuel, then #2, then #1. Dual fuel lines are a good idea though. I feel a bigger possibility for burning #3 cyl is it is where the rear seal is, so there is a greater chance for a large air leak there, and thus lean out on high loads. If the front seal is leaking, there is still the front cover which is sealed and would reduce total leakage to #1.

  9. #9
    LT1 hit the nail on the head. I also think most rear cylinders go lean because of a leaky rear seal , besides the usual oil line problem. He is also right regarding the rear cylinder getting fuel first. I would double check that ur carbs r still working properly too.

  10. #10
    SMLaker's Avatar
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    Need to go back and look at this again. Looks like y'all are saying what I thought was the feul delivery line is actually the return line? Fuel delivery line hits Accelerator pump first then eh?

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