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  1. #1

    Did I rebuild my carbs wrong? GP12R

    Boy do I feel really really dumb for not following directions.

    So I rebuilt my carbs about a month ago, and unfortunately did not realize the Mikuni Check Valves were to be installed one-way with the blue marks facing up/out. I have no idea if any or some are right or wrong at this point. My ski is kinda tough to coldstart and has had low compression all summer (~90psi on all three) but once its running it runs all day and re-starts beautifully. The plugs look good and the tune looks to be alright as well. Maximum RPMs are at 6900 which is kinda low for the mods it has but probably pivotal based on compression.

    Now I'm stuck with a dilema. Should I be able to replace just the valve disks and the grommets without damaging the rest of the rebuilt pumps? The cost for the parts is about $40 total compared to about $150 for three full rebuild kits.

    I guess another option is run it while its still running great until the season is over. I plan to do a top-end refresh which means the carbs probably gotta come off anyway. I'd of course prefer to not rebuild the whole carb, but just fix the valve issues.

  2. #2
    ok I'm pretty resourceful.

    I hooked up a fuel pressure gauge to my return line and will try/see next time I get on the water what my return fuel pressure is running at idle and wot. It should be 2psi idle and 6psi wot.

    I've got 1/8" drilled carb returns and a 95 jet in-line return. The gauge is hooked up between the carb return and the 95 jet. Just cranking over it get .5psi of pressure without it starting; 10 seconds cranking.

  3. #3
    elmatta's Avatar
    Join Date
    Sep 2011
    I don't believe the marks on the check valves matter. They are simply pieces of plastic. Your problem lays elsewhere. How many hours on your ski? Fully charged battery? The pump is on the opposite side. You can replace the checks and grommet without touching the pump.

    90psi is a problem in itself that should be addressed. I would call it a season and take a look at your top end and maybe the crank depending on your hours. After I did my own carb rebuild, my ski starts instantly or first 3-5 seconds, even after a week of sitting. This might be only me but I seem to flood when I dont shut off my gas after riding and starting it the next day or so. I usually shut it off then open the gas cap before storing.

  4. #4
    If my fuel pressure looks good on the water I'll care less about the check valves.

    Yes I plan to at the very least re-ring during the off season. The engine has been running the last 35hours with low compression. Ski has 90 hours 12 years old. it's tired but not catastrophic yet.

  5. #5
    This is how I run a jetski shop in the desert nmpeter's Avatar
    Join Date
    May 2008
    Elephant Butte New Mexico
    Quote Originally Posted by elmatta View Post
    I don't believe the marks on the check valves matter. They are simply pieces of plastic. .
    with all due respect you are dead wrong on that. dead wrong.

    the mark indicates the possible curl on the check valve and there are very specific instructions on which way is "up"

    A very quick search on the topic would have directed you to this sticky by my carb Guru Oside Bill (R.I.P. there Bill) on the Yamaha side:

    I've seen a few cases where incorrectly installed disks on yam 1200's appear to have been creased during operation.

    It's a mistake you'll only make once.

    Even if it all seems right out of the gate..the disc WILL curl..causing you lots of headaches down the river.

  6. #6

    Join Date
    Apr 2010
    Greenville, NC
    I have the same problem with mine. Will the check valves being installed wrong cause a drop in rpm's
    Last edited by 992door; 08-18-2013 at 09:41 AM.

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