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99 1100 zxi cv carb adjustments

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  • #16
    Re: 99 1100 zxi cv carb adjustments

    I have been looking into this a lot lately because I have these carbs and I just can't believe that there is no way to make adjustments. The carburetor itself is pretty simple:

    -fixed high and low speed jets
    -fixed needle valve and seat controlled by a float arm and diaphram
    -a needle jet and seat controlled by a diaphram/spring and venturi

    In low speed including idle, there is not much to adjust other than the sychronization of the trottle valves and the idle screw which basically just sets the amount of air flow. The low speed jet goes through a bypass and is fixed. There should be a low speed screw adjustment and I have read that there is one hidden that you have to drill out, but I have not tried.

    In high speed (WOT) everything is controlled by the balance between the venturi action, the diaphram/spring and the needle valve/seat. The venturi speed is controlled by the trottle valve position and the design of the air cavity which is why people highly recommend you do NOT remove the choke plates when installing primers. This undoubtably disrupts the designed air flow and likely makes the ski run lean. The spring is the key part here. If the spring has a lower spring constant, then it will open more readily with less air and allow more fuel (richer condition). The opposite is true for a spring with a higher spring constant. In my opinion the carbureator should have been designed with a thumb screw in the back of the case holding the spring in place to adjust its tension.

    I believe (just my engineering opinion) what they did in designing the diaphram/spring is leave adjustment for vacuum control. The venturi pulls air out of the back of the diaphram holding the needle valve through 2 small holes near the base of the needle. That creates a vacuum behind the diaphram and pulls the needle backwards releasing fuel from the jet. The vacuum cavity behind the diaphram has a hole/slot through which air can come in to relieve the vacuum. This hole is part of the rubber carbureator stack inside the air box under the flame arrestor. (See attached picture)

    So, what I believe you can do is rotate the rubber stack on each carburetor to adjust the amount of air that can enter the vacuum cavity. If for instance you were to rotate them to the mostly closed position, then the vacuum cavity will be mostly sealed off. This means that the venturi will have an easier time creating a vacuum behind the spring and the needle jet will open more easily making the ski run richer.

    Seems to me that this is the small adjustment that the designers left in the carburetor.

    The air slots are on the right hand side of the picture and the openings can be controlled by rotating the rubber stack.


    Click image for larger version

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    Last edited by dlofgreen; 09-07-2012, 12:16 PM.

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    • #17
      Re: 99 1100 zxi cv carb adjustments

      After making this adjustment for cylinder #2 (the one running lean) I personally believe it made a difference. I rotated the carb stack (duct) about half of the maximum allowable. Ran the ski for the 3rd day (after 2 days of break in) up to WOT (~7500 rpm) and everything felt great. The ski was fast and responsive (~60mph).

      Here is a picture comparison of the plugs, I will let everyone form their own opinions.

      Click image for larger version

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      • #18
        Re: 99 1100 zxi cv carb adjustments

        Sorry for the long reply, work has been crazy and I haven't had too much time to mess with my ski that past couple weeks. I did get larget pilot jets, and that seems to have greatly helped my bogging issue. It still seems to bog a little when its on the water though and I run WOT. It takes at least 30 seconds to catch up. When it does I can feel somthing open up, I'm assuming thats the CV slides? When they do it takes off and does around 60 at about 7500 rpm like yours. The next time I take it out I'll try rotating the stacks on the intake box like you said. It almost feels like somthing is sticking, I was thinking it could be a pop off pressure issue as well. Since I put the larger jets in all three plugs look alike, so I think I corrected the lean condition on my #3 cylinder. It sure was nice to ride that thing and have it actually go fast though. just out of curiosity how was yours on fuel? I burned through a tank in a little under two hours, seems like alot but it sounds like thats about normal for these skis.

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        • #19
          Re: 99 1100 zxi cv carb adjustments

          I would verify the lean condition by looking at your spark plugs. A lean cylinder runs hot and makes your plug very dry and white/grey. If you think your slides might be responding slowly I would say it's either that you have a small air leak in the intake that is redirecting air away from the Carb/venturi or your slides are dirty. You could clean them (both parts). I would wash them and use acetone to get any gunk off. Wiping them with 2-cycle oil would not hurt either.

          I don't really trust my gas gauge but I would say we burned around 10 gallons in 5 hours that's probably 3 hours of operation

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          • #20
            Re: 99 1100 zxi cv carb adjustments

            One thing that caught my attention as I re-read your post. You said the pto cylinder looked clean but you said like new. Did you mean new like out of the box silver new? If so I don't think your plug is firing. Did you check spark? You can pull all your plugs and hook them up to the wires one by one. Rest it against the body of the engine and turn the ski over. Verify that each sparks. Be sure to cover the head holes so you dont get fuel escaping next to your plug.

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            • #21
              Re: 99 1100 zxi cv carb adjustments

              I do not often say this, but I am going to eat my words on the CDCV carbs. After piston failure on my rebuilt engine I decided the ski must be running lean. I rebuilt the engine again, but this time I installed the CDK-II carbs. When I pulled the plugs this time after break in, I could clearly tell the difference. The engine is definitely NOT running lean now, in fact, possibly even a bit rich.

              I also did the following:
              1) I am going to lean down to 40:1 not 50:1 as I think this is too oil lean
              2) Since I am running pre-mix, I removed the front bearing in the stator that translate rotation from the flywheel to the oil pump. I decided that all these bearings were doing was adding heat to the stator.
              3) I took the front cooling cover off the stator and cleaned things up well since there was some corrosion there (something I should have done last time)
              4) Used SBTs pistons with the carbon coating for added heat resistance since my heads are milled.

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              • #22
                Re: 99 1100 zxi cv carb adjustments

                Quick follow-up for anyone interested...

                After rebuilding the engine again with the 4 corrections above, I took the ski out for engine break in. The ski ran GREAT! 3 days of breaking the engine in, got to WOT and everything is smooth and fast. I have been out about 4 times since with zero problems.

                I have pulled the plugs to look at them a couple of times, and it is obvious to me that there is a difference. Before my pictures showed they were dry and brown (not grey or ashy) but now they look slightly wet and almost shiny brown. Tops of the pistons look good too.

                I am positive that replacing those CDCV carbs with the CDK-II ones did the trick.

                The last thing that I did was remove the primer gas line from the supply to the return. I decided that if there was any way the primer could let go or put some air into the line that I would want it to go into the return not the supply that then fed the carbs directly.

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