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  1. #1

    1997 gp 1200 carb problem? (EDIT: COMPRESSION PROBLEM)

    So I have used my GP1200 maybe 10 times over the past couple years since I have bought it from a friend. Until last weekend it ran great (after I did a lot of basic repair work). It was running great last weekend on Saturday, but on sunday, almost immediatly after starting it up, it was running really rough. It would idle pretty well but when throttle was applied it would start to speed up and then bog down. It would move at a slow pace and almost get onto it's plane but just sit there. A strange sound would come from the engine compartment that sounded like air rushing in and out of a valve. It seemed as if maybe one of the cylinders wasn't firing. If I feathered the throttle from about 25% to 50% for 5-10 seconds it would start running better and then it would take off and i run great. Full power and everything. I could reduce throttle and idle and stuff and it would continue running well unless I let it idle for more than a few minutes. If it gets turned off the whole process needs to start over again.

    Same thing this weekend. I decided I should stop running it until I figure out what the problem is, just incase it's something serious. After I got home Saturday (this week), pulled all three spark plugs. The middle spark plug had some kind of smoke/vapor/steam or something coming out of it and the middle spark plug was obviously dirtier than the rest.

    To me it seems like a carb problem. I have rebuilt carbs for my motorcycles plenty of times but never anything for a 2-stroke. My other concern is a blown headgasket, but it seems like after running it for two days and the problem being internmittent, I would have had more issues.

    Any ideas? I am temtped to order a carb rebuild kit, but before I waste the money I was wondering if anyone had some tips or suggestions. Is rebuilding a 2-stroke car the same as a 4stroke? The aftermarket flame arrestors the original owner installed seem to dribble the oil through teh top, down into the carb... seems strange to me.

    thanks!
    Last edited by chadbobb; 08-19-2012 at 02:07 AM.


  2. #2
    Yamaha artisan Cutlass's Avatar
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    Its probably carb related. But I would do a compression test first, just to make sure the engine is in good shape before digging into the carbs.
    When you do the carbs, only use genuine Mikuni carb kits. The cheaper aftermarket kits don't work. You'll need 3 kits (one for each carb). You should also order the 3 carb to rack gaskets and the carb rack to intake gasket in case the old ones aren't reusable.
    And you'll need a pop-off tester.
    These carbs aren't too bad to do. There is a bit more work (and a few weird things) compared to most motorcycle and atv carbs I've seen.
    Here's a link to the service manual to these carbs. Read up and familiarize your self with them.

    http://www.mikuni.com/pdf/sbn_manual.pdf

    Read up on them on here too. there's quite a bit of info on these, including reliability mods, updates, and improvements you can do

  3. #3
    needles and seats arent included in the rebuild kits. they have to be bought seperately. might as well replace them if your going to rebuild the carbs

  4. #4
    is this one of those things where I might be able to get away with just cleaning them? I have manged to never need to buy a rebuild kit for my motorcycles by very VERY carefully taking them apart and cleaning them and not tearing any gaskets or o-rings.

  5. #5
    silvercrxsir's Avatar
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    Have you done a compression check,I had a lower crank bearing go out which put a small hole in the lower case the motor never made any sound like it blew up and compression dropped in the first cyl from 140 to 90 psi,the air rushing sound I had was from the hole in the case,and it would only run on the primer due to it being the only fuel source since the engine had no vacuum.I would start by first running a compression check just to see if the cylinders are ok,then followed by a leakdown these to make sure that you have no air leaks because its vital for fuel flow and its an older motor so by checking this it can prevent a possible future failure.

  6. #6
    Yamaha artisan Cutlass's Avatar
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    Quote Originally Posted by chadbobb View Post
    is this one of those things where I might be able to get away with just cleaning them? I have manged to never need to buy a rebuild kit for my motorcycles by very VERY carefully taking them apart and cleaning them and not tearing any gaskets or o-rings.
    Yes, possibly. I don't necessarily recommend it. If its been quite a while since the last rebuild, you're better off with fresh gaskets, seals, and orings. Same story if they used aftermarket kits or you find creased check valves or any other problem.

  7. #7
    Quote Originally Posted by silvercrxsir View Post
    Have you done a compression check,I had a lower crank bearing go out which put a small hole in the lower case the motor never made any sound like it blew up and compression dropped in the first cyl from 140 to 90 psi,the air rushing sound I had was from the hole in the case,and it would only run on the primer due to it being the only fuel source since the engine had no vacuum.I would start by first running a compression check just to see if the cylinders are ok,then followed by a leakdown these to make sure that you have no air leaks because its vital for fuel flow and its an older motor so by checking this it can prevent a possible future failure.
    No, I can when I get my compression guage back from a friend this friday. I can't do a leakdown test though, maybe in the future. I'll have to figure it out without one

  8. #8
    Ok, Update:
    I was about to start tearing the carbs apart when I decided to have my friend get me my compression gauge before doing so just to verify. Turns out I have 120, 60, 115 PSI according to my cheap gauge from front to back. Obviously there is something seriously wrong with the middle cylinder.

    Again, I am not too familiar with 2 strokes but from what I understand it should be fairly straight forward to repair. Please let me know if my thoughts are somewhat correct. I need to determine if the piston rings are worn, the piston is worn, or if i have a blown head gasket.

    what is the best way to go about that? I did end up removing the carbs just to see if I could see any damage around the head gasket visibly... I knew it was a long shot but I went ahead and removed them. I can post those pictures if anyone thinks they might help.

    thanks!

  9. #9
    Yamaha artisan Cutlass's Avatar
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    Next step is to remove the head. You'll probably see something wrong with the middle cylinder then. Next step after that is to remove the cylinders. All three come off at the same time in one big block.

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