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  1. #1

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    Issues with pistons

    I have a 97 gp 1200, it was the old one at Jacksonville FRS. I was running a 39.5cc domes and broke the 2 front pistons right by the exhaust port. I assumed detonation. I am now running 84.25mm Pro X with a stock head at 125 psi.I tuned with a tiny tack but could not get the rpms perfectly still. I got them within 40 rpm up and down, but that was the best i could do. After about 10 hours of run time I lost the front piston, with the piston broke right by the exhaust port. My mods are just riva intake, and no chokes. Upon dissassembly I found the line from the front cylinder fitting to the middle fuel pump had a razor knife cut in it. Could that do it? Oh and is it safe to run 39.5cc domes? And if I can go with 39.5cc what should my squish be?
    Last edited by SLOW65U; 04-02-2007 at 09:48 PM. Reason: afterthought


  2. #2
    Duke's Avatar
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    Can you post some pictures of all 3 cylinders? The piston wash might help with diagnosing the problem. Do you have the Riva CDI? What were your rpms give or take 40.

    I think I talked to you at the FRS on the ramp.

  3. #3

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    It sounds like you are lean on those cylinders where the piston keeps melting (assuming that the top edge of the piston is melting, not cracking or breaking off, right in front of the exhaust port). You need fuel and oil to lube the piston, as well as keep the crown cool. It sounds like you've got enough to lube, but not enough to keep it cool.

    If your pulse line failed (the one with the razor slit), that could cause it for that one cylinder. I just put new pulse lines on mine ($15 is not a whole lot for peace of mind).

    That'd be my guess.

    What was your compression with the 39.5 cc domes?

  4. #4

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    well the edge by the exhaust port did break off. The pistonwash looks perfect. according to every pic I have seen. Rpms were roughly 6720-6740. When the engine seized I was on a long ride. I let the motor cool and it would be ok for a few mins. Then it would die again. Let it cool then it would run again but shorter time than before. I repeated till I got to the dock. Then it would only run for 10 sec. compression was 153lbs with the 39.5cc dome.

  5. #5

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    Did they look something like this? (bare with me, this is my first time trying to post pics)
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  6. #6

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    That was all 3 of my cylinders the summer before last. If you look close, you can see where the edge of each piston melted right in front of the exhaust port. I thought each one had a little chip broken off too, but it actually melted away.

    Do your pistons look similar?

    Judging by my wash (or lack of it), I would think that I was running rich.

  7. #7

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    the edge keeps breaking off just like your pic. But my pistonwas looks perfect. black on top with clean spots the size of a fingernail by the ports.

  8. #8

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    Here's my thoughts: Both of the pulse lines for the front 2 carbs come from the #1 cylinder. A cut in either one of these lines will disable both fuel pumps on both carbs and cause both cylinders to run lean (since they are essentially linked together) and cause the damage that you describe.

    If you are certain that one of these pulse lines has a hole (or slit) in it deep enough to cause this line to fail, then I think you have found your problem. I would replace all of your pulse lines and check the 'check valves' in the carbs and give them a good cleaning while you are doing your top end rebuild. I'd also replace your fuel filter if you haven't done that this year yet either.

    Any idea how that line got cut? (so you don't have to worry about it happening again?) It seems like those lines would be protected rather well from any cuts or abrasions considering where they are in relation to any moving parts.

    I'm also surprised that you had that good of a wash after only 10 hours (I'm not saying that I'm a wash expert by any means). My pistons above had at least 20, if not more.

  9. #9

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    Here is some camera phone pics
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  10. #10
    Duke's Avatar
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    Superd, IMO you are rich on jetting but looks like your having a deto problems. What type of head, compression?

    Slow, You are lean. Don't know the cause but that wash indicates lean, lean, lean. Does not seem to be any sign of deto on the 2 good pistons but the picture quality doesn't help.

    My .02 .

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