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  1. #1

    gp1200 carbs wont stop dribbiling

    I recently rebuilt the carbs on a gp1200 and I canít get the carbs to stop dribbling from the venturi. I installed new 1.2 n/s and set the pop off pressure to around 64 psi which helped some but the still dribble. On the bench the n/s holds 64 psi without leaking. I also doubled up on the anti-siphon valves / valve body check valves. I think that I am pretty much maxed out on the pop off pressure. Is there anything else that can be done?
    I also helped my friend rebuild the carbs on his wave venture 1100. The pop off pressure is set to only 40 psi and the carbs donít dribble. These are similar engines. Why does the gp seam to be so much more susceptible to this problem?
    Any input would be greatly appreciated.

  2. #2
    onefastgpr619's Avatar
    Join Date
    Sep 2009
    San Diego, Ca
    send Osidebill a pm. He will answer all your questions.

  3. #3
    Dave Sharp dav_dman's Avatar
    Join Date
    Aug 2005
    louisville Ky/so. IN
    mine dribbled right after rebuild for about 3-5 hrs then stopped. Beats the hell outta me.

  4. #4
    I have about 6 hours on the ski since the rebuild. I will check to see if they stopped dribbiling on their own.

    I should also add that the ski seams to run fine otherwise. I does not load up at low rpm and the throttle response seam good to.

  5. #5
    ok, first off i have had a bit of experience with this problem.

    i rebuilt my carbs 2 times and it did not solve the problem.

    i doubled up my checkvalves, and doubled up my pump valves because of the gp's ability to crease them. did not solve the problem.

    i then thought it must be a fuel pump problem, so i bypassed the internal fuelpumps and used an electric fuel pump to supply pressure more consistantely. this did not solve the problem.

    i then removed, inspected the reed cages, no problems. this didnt solve the problem, so i replaced the reeds, did not solve the problem.

    i then changed the n/s to different ones for different pop off. did not solve the problem.

    basically everything i threw at it didnt fix it.

    honestly. i had just had the engine rebuilt and i was ready to accept "thats the way it works" dispite my perfectionist attitude.

    turns out the idiot who did the engine bore/hone thought 10-15 thou piston clearance is cool. NOT
    i cannot beleive the ski would even hit 55-58ish with this terrible engine work! which is why i did not suspect the bore job.

    combustion gasses were leaking past the rings/pistons from the idiots bore/hone job, despite compression numbers coming up excellent!
    so when the gasses leaked by, the case pressure spiked, this caused overloading on the fuel pumps (more about that later) and also caused pressure spikes in the intake manifold.

    beleive me, as perplexing as it was, this was the cause.

    I rebuilt the motor with a proper bore/hone job and excellent piston clearance setup. it never dribbles anymore except for a minor bit after a choked startup.

    i suspect there are 2 - 3 things setting it up for the gp's to do this. one is the restrictor design in the carb fuel pumps. its just pretty damn restrictive. this causes the fuel checkvalves to crease from over pressure in the pumps. this is why most gp tuners recommend to drill out the restrictors and install a single return line restrictor with a fuel pressure of 6-7 psi at WOT.

    the second thing is the damn shakiness of the 65u motor. i beleive its got alot to do with the ignition timing at idle. because of the lack of sensors the older engines are limited to a 2d ignition curve (rpm vs timing) we dont get an engine load signal, so we are forced to compromize our ignition timing to maximum load values. not ideal.

    lastly its got alot to do with piston ring blow by, wreaking havoc with idle carburetion and the pulse style pump.

    how many hours does your motor have? perhaps its time for a top end freshening?

    i would recommend doing the single return restrictor modification, and making sure that your reeds are in good shape too.

    An excellent telltale hint for combustion blowby is the pumps plastic diaphragm. if there is heat marking there you have a problem. I noticed mine early on due to the clear fuel lines i had used for the pulse lines to the carbs, the 2 cylinders with the worst off bore situation would burn up those lines.

    Coming from my 4 stroke turbo car tuning it was alittle annoying to adapt to the 2strokes (and especially the gp's engine) needs, but certain things are important/needed

    1. the idle needs to be set slightly/somewhat rich. you do not want it too rich but rich enough to totally avoid backfires, leanburning. on a 2 stroke this leanburning idle is needless heat. although idling rich makes worse emissions, it pretty much needed to make the gp motor happy. set the idle speed as factory wants, as well as the idle mixture screws AKA low speed adjusters. You can play with them up to 3/4 turn to improve things but dont go overboard, make sure you have crisp throttle response!

    2. The richness at idle also assists in crisp throttle response. These carbs are primative and do not have a power valve or an accelerator enrichment device. so that rich idle/lowspeed comes back to make this a happy beastie for acceleration.

    3. The rich idle again helps prevent case pressure spiking and ruining the pump check valves. CRITICAL that you dont lose fuel pump capacity! so in addition to the rich idle, make sure your check valves are A OK and i really recommend the pressure restrictor mod.

    4. The factory settings for the carbs are QUITE good for the factory motor. You could improve upon it with osidebill's recommended setup but for the most part its good enough. Bills setup is well documented on this site. search gp1200 carb settings. you WILL find his settings posted.

    5. The 65u engine loves to shake at idle. its probably best to set your idle with the ski in the water on the trailer strapped down. adjust the idle so its near the factory recommended idle speed but also as smooth as it gets. that shaking does not help the needle/seats do their job nicely.

    So in short, check the engine's internal condition, a compression test is good but may not yeild 100% information.
    Check the reeds, and do the restrictor mod. 1-2 psi at idle. 6-7 at high rpm wot
    Last edited by the_natrix; 06-28-2010 at 05:10 PM. Reason: wanted to add more

  6. #6
    Dave Sharp dav_dman's Avatar
    Join Date
    Aug 2005
    louisville Ky/so. IN
    1 HE said!

    [NICE write up, Natrix]

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