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  1. #1

    Rebuilt GP1200R - Almost there!

    Just went through a complete rebuild for the first time from scratch on a GPR I picked up in pieces. After replacing the CDI, it is now finally running. It actually starts up and runs pretty well, but a few issues came up. I've got 120 on the middle and rear cylinder, and just 80 on the front. Also have some minor backfiring. I pulled the head and the piston looks great and the rings fit perfectly to the bore during assembly.

    I'm curious, is it likely that bad reeds on that cylinder would cause that much of a drop in pressure? I bought some used ones off ebay because I didn't have the cages or anything, but might just go ahead a get a complete set of new oem reeds. I completely cleaned the power valves and used a green brillo pad to remove the carbon buildup. I'm hoping there's no issue there. I'm still planning on going with flame arrestors and working on the jetting, but I did rebuild the carbs with Mikuni parts. At this point, I'm at least given hope by the thing running!


  2. #2
    leejax01's Avatar
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    Are the piston rings new? Is the cylinder in spec? Is the piston new? Is the piston the right size ie A,B,C,D for the stamped cylinder? Is the locator pin for the piston ring all the way out? They can push in and cause the ring to spin. Had one like that and caused low compression.

  3. #3
    I used a WSM top end kit with all new pistons and rings. The pistons didn't have any letters, but I did make sure the arrows pointed toward the exhaust ports. I reused my good cylinders and picked up a cylinder in great shape off ebay. Nikasil still looked normal thickness with no scoring on all cylinders. If all else fails, I maybe have to check those rings. Just hoping the easier solution might be a possibility I guess.

  4. #4

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    Low compression is is either rings, not round cyl, crack in head, or bad head gasket/surface uneven. Which brings up a point: Not all cyl are the same height, especially if all are not from the same original engine. Slight differences year to year, plus if any have ever been surfaced or overheated things will change. The thing is if they are not all the same height, they will not be flat in reference to the head and the head gasket then can leak.

  5. #5
    As suggested above, it turned out to be the rings. I pulled the motor and disassembled down to the suspect cylinder, and found the rings to be stuck into the ring grooves, if that makes sense. I removed them, dipped them in oil and reassembled. Now I'm showing 120 in all three cylinders. There may be some hope for having some fun this season yet!

  6. #6

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    Are you sure the rings are in right? Sometimes they have a taper on one side and have to go with the correct side up. That would cause a bind if they were upside down.

  7. #7
    Went on the maiden voyage yesterday, and it was nice to see everything work out as it should. I ran it for a little bit on the trailer and then out on the lake for about an hour. The only issue that seemed to come up was the front pissers don't seem to be making any output. It may have been running a little lean too. I pulled the plugs after and there was a bit of dry carbon buildup, but maybe that's fine. I tried to avoid hanging at a constant speed for any bit of time for breaking in. As much as I hate to pull the stinger and carbs once again, I think I may need to fix the carb jetting to be safe. From what I understand, 110 pilot/125 main/1.5 needle/95g spring is the way to go for a mostly stock setup. As for the pissers I guess I will try to see if there's any blockage in the lines, and try covering up the main for a sec so it forces out the others?

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