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  1. #1

    gp1200r cylinder wall

    So I bought a gp1200r that had a bad front piston, but the guy incoluded a nice new piston with it. This is an sbt engine too btw. Apparenlty his kid ran it in the mid range too long and burned a piston because it was running to lean. Ive heard thats a problem with these carbs. But before I rebuilt the carbs and after I replaced the piston I ran at a constant 1/4 throttle for a good hour. I did not check the compression after, I just took it out of the water. I know now that the throttle needs to be played around with and not just sit at one point. Since I had to order rebuild kits for some other skis I went ahead and ordered some kits for this one. I cleaned the carbs and followed OSidebills carb upgrade guide with the rejetting. So I took it out today to finish the break in progress at around 1/2 throttle for an hour, again at the time I did not know I was suppose to move around with the throttle in short bursts. After an hour I went to 3/4 throttle and it was running fine through 3/4, but as I got closer to full throttle it seemed like it was losing power. Then it would not go past 5k rpms and the only way to get it past was to go 3/4 throttle. So fast foward and I get home do a compression test and the cylinders read 150/150/60 back to front. The 60 psi cylinder was where I replaced the piston. After taking the head off it was clear the piston was burned pretty badly and melted on the side closest to the exhaust. It even made a hole though the piston. The cylinder did not have too much damage, all it seems it that the piston melted off some of it on the cylinder wall (see pics) So I am wondering, why this would have happened. When I did I not break it in correctly could it have caused it to burn up like this. Or maybe since the carbs were a bit dirty that running the ski in the low range for a constant hour made the ski run too lean and the front cylinder melted. Combination of both? I will post some more pictures tomorrow of the piston. Also I converted the ski to premix and I was running it at 32:1. Any thoughts? And will I need to replace the cylinder, or could I somehow get that melted on metal off of the wall? Thanks Click image for larger version. 

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    Last edited by jetskifan23; 06-06-2015 at 09:42 PM.


  2. #2
    Here are some pics of the piston

    Click image for larger version. 

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    So the question is where do I go from now? Is there a way to get that metal off the cylinder wall? Should I take the engine out and open up the crankcases in case there is metal left over from this piston and the last? How to go about figuring out why this happened again, me not breaking it in correctly or carbs were too lean or maybe an air leak in the front seal? The motor itself does not have many hours on it. Can I buy a cylinder with a piston that has 150psi and throw it on if the cylinder is no good? Thanks for looking

  3. #3
    Muradic Acid will take the aluminum off the cylinder walls on plated cylinders

  4. #4
    Thats a steel sleeve. Be careful using acid on that. It will eat the sleeve as well. You need to mic that cylinder to see if it was bored the correct size either that or you have a issue with that front carb.

  5. #5
    pics of the old piston, I dont think any loose metal came off of this one just looks really burnt Click image for larger version. 

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  6. #6
    That piston looks good

  7. #7
    Quote Originally Posted by ctct2118 View Post
    Thats a steel sleeve. Be careful using acid on that. It will eat the sleeve as well. You need to mic that cylinder to see if it was bored the correct size either that or you have a issue with that front carb.
    It may have been that front carb since the ski had sat for awhile and I ran it like an hour straight when I was breaking it in. So how should I go about fixing the cylinder if I cant use acid? Does this mean I could just resleeve it myself? The guy I bought it from said he removed the piston because it had low compression aparenlty though.. Maybe I should try to throw it in or something and test the compression.

  8. #8
    The top ring is harder to move around then the bottom one when I try to spin them.

    Edit: got the top ring to move around now and lined up with the marker

  9. #9
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    the dome of this cylinder was pretty damaged as well. Ive read this can be fixed, but am I better off buying a used head online? Will this need to be fixed in the first place?

  10. #10

    Join Date
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    Jetskifan I might have missed it in your posts but other than swapping out broken parts have you changed anything to fix this?
    This looks like it has over heated on the exhaust side , exposed the ring causing the ring to break sending shrapnel through your engine. IMHO there is an under lying issue here which needs a further look at unless you address this it is likely to happen again. My guess was fuel.

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