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  1. #1

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    Carb Block Off Plates and Air/Fuel Separators

    Anyone have any experience with this setup on a GP1200?

    I have a Group K Hammer kit with triple pipes and the carbs were modded by removing the fuel pump sections and installing block off plates.
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    The carbs have the original small (.020-.030") fuel return restrictors. I am also running an early NOVI(?) Air/Fuel separator with numerous hose fittings on the sides plus the obligatory fittings on the top and bottom. I've searched and searched to try to find documentation for a similar setup without success.

    I've flushed the fuel system, I've ordered a new high flow Mikuni DF62 "pentagon" pump to replace my original Bombardier fuel pump, I'm rebuilding the carbs and I want to optimize the fuel delivery system for reliability and consistent performance.

    The engine ran great originally but then life intervened and the ski sat for several years and all of the urethane fuel lines disintegrated and left a T fitting with a restrictor jet laying in the bilge (you can just see the Y with the yellow ty wraps at the bottom of the pic). I know this was in the fuel system somehow, but I can't find any documentation as to where. I suspect the restrictor jet was in the vapor return line (the top-most fitting on the air/fuel separator) going back to the tank and T'ed into the carb fuel return line.
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    I have read here about multi-carb systems where the restrictors in the carb fuel pumps are drilled out and a single restrictor is added in the common return line. The restrictor (usually a main or pilot jet) is sized with certain pressure targets for idle and WOT acheived through adjusting the common restrictor size. However, these are all systems where the carb fuel pumps are retained. These same systems are sometimes enhanced through the use of an air/fuel separator, such as the one sold by Group K http://www.groupk.com/sep.htm, but the plumbing requirements would seem different based on whether you are using only remote fuel pump(s) vs. the pumps integrated into the carbs.


    OK, so here are my questions:

    1. Does the Needle/Seat ("N/S") need to have a few psi of fuel pressure applied to the fuel inlet to ensure adequate fuel flow, or will the carb "suck" the fuel it needs with just a few inches of gravity assisting the flow across the N/S?

    1a. Put another way, do I still need to target 1-2 psi at idle and 6 psi at WOT with this setup, or is it sufficient just to provide a volume of low-pressure (less than 1 psi) fuel to the N/S and let the intake signal draw the fuel through the carb.

    2. If the 2-6 psi range is required, can I achieve this on the pressure side by putting the restrictor in the Air/fuel separator vapor return line and retain the stock .020-.030" restrictors in each carb? (All three carbs are fed from the same remote pump via the fuel/air separator and should therefor all see the same inlet pressure.

    Lastly, I've been advised by Group K to bypass the fuel selector to eliminate any reduced fuel flow caused by going through the selector. I'm concerned that I may flood the engine with fuel after I shut down if there is no fuel cut off in the circuit and there is pressure in the fuel tank.
    3. Will a 60 psi pop-off pressure (recommended by Group K for this engine) prevent any fuel seepage, provided the N/S are in good condition?

    Thanks in advance for your help and suggestions.
    Mike
    Last edited by TwoGP12s; 10-10-2010 at 01:31 AM.


  2. #2
    Moderator OsideBill's Avatar
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    Run the restrictor in the Novi separator to obtain pressure and cap the return fitting in the carb. Run the feed from the Novi Separator to each carb. Cut a tee in one of the feed lines for a gauge to monitor pressure. Pop-off does not need to change the fuel pressure will not affect it. Use a jet to obtain 1-2 PSI idle and 5-7 PSI wide open throttle.

  3. #3

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    Thanks, Bill.

    I was hoping you would take a look at this posting and leave a comment. I'll do as you say and report back. I'm still waiting on the new pump and a rebuild kit.

    Thanks again,
    Mike
    Last edited by TwoGP12s; 10-10-2010 at 01:31 AM.

  4. #4
    Moderator OsideBill's Avatar
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    here is one I made same principal as the Novi, Ignore the first part it is instructions for mounting carbs with reeds on a GPR the fuel seperator instructions are at the end.
    Attached Files Attached Files

  5. #5
    smacota1's Avatar
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    Bill,

    Here is an idea i am thinking about. I plan to use 6MMX50MM (or adjust to needed length) studs to align the carbs using the four upper and lower bolts between the carbs . I feel it will make installing and removing carbs easier and will eliminate the possibility of stripping the bolt holes out when you have to remove the carbs for a lot of tuning.

    This is a great pdf.

  6. #6

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    Sep 2010
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    Lake Travis, TX
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    Thanks for the PDF! Except for the twin fuel pumps, that is exactly the setup I intend to run.

    Group K suggested I bypass the fuel selector as it may limit my fuel flow. Is there a way to mitigate the risk of fuel entering the engine after shutdown between rides, or during the off season? I was thinking of using the fuel selector as a vent in the OFF position to depressurize the fuel system. Is this wise? Is it necessary?

    Mike
    Last edited by TwoGP12s; 10-10-2010 at 01:32 AM.

  7. #7
    krispaintballz's Avatar
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  8. #8

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    Yikes, Kris.

    That's way more complicated than I had in mind. Then again, I'm sure you're making way more horsepower.

    Here's my setup:
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  9. #9

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    Sep 2010
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    Lake Travis, TX
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    Thumbs up Got My Ski Back On the Water Today !!!

    First off, thanks to all with your suggestions, and help.

    I finally received all of my rebuild kits, gaskets, Mikuni "pentagon" fuel pump, fuel filter, various fittings and fuel line. Got everything assembled and started the engine (after a few operator errors along the way...). I ran 5/16" fuel line for the primary fuel circuit between the tank and the fuel pump. I also changed the fuel filter to a model with 5/16" barbs in and out (these are the black fuel lines in the pic, below left). The fuel pump has dual outlets, so I used 1/4" line for both outlets into the fuel/air separator and then ran dual 1/4" fuel lines to the carbs (These are the two the blue hoses on the left side of the fuel pump in the pic, below right, that deliver fuel to the F/A separator. The black hose is the 5/16" fuel inlet hose. The blue hose on the bottom of the pump is a very stiff piece of Aeroquip fuel line for the pulse line from the engine).
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    It ran much smoother at idle than I can remember, and best of all, it no longer dies after a few seconds of WOT. With my original 60 jet in the vapor return line, and all of the carburetor return fittings capped off, I was seeing 3-4 psi at idle and 7-9 psi at WOT.

    Since I needed to decrease the fuel pressure, I thought I would try uncapping the return lines and see what my pressure would do. My reasoning for this move was to reduce the chance of vapor lock and circulate the fuel so the fuel in the carbs would be a little cooler. I plumbed the carb return lines together like stock and then tee'd the return line into my vapor return line to the tank (this is the grey line at the far left in the pic below). Now my pressure was a steady 3 psi at idle and 7 psi at WOT. Its better, but I will try some larger restrictor jets in the vapor return line.

    While I was reconfiguring my fuel system I also took the time to redo my primer circuit. The original lines were getting hard and inflexible. Even worse, I was sourcing the primer fuel from the main fuel inlet, which is a possible source of air in the fuel if there is a leak in the primer system.
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    And yes, I'll replace the plastic T w/ brass when I place my next McMaster Carr order.

    Yet again the Green Hulk forums have saved me from my own ignorance.

    Thanks again,
    Mike

  10. #10

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    Sep 2010
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    Thumbs up Dialing In the Jetting

    It's hitting hard now and really launching when it comes up on pipe! I'd forgotten how violent the acceleration is on this ski...

    I ended up with a 100 jet in the vapor return circuit which gave me 2psi at idle and 4-5psi at WOT. I suppose I could try an 80 for the vapor restrictor, but it didn't seem worthwhile at the time.

    It's a bit lean on the bottom end so I kept enriching the idle circuit to cover up the lean transition and it kept getting better. I've ordered some jets so I can enrich the lo-speed circuit without compromising my idle mixture. It took a full half turn on the adjusters to minimize (but not eliminate) the bog from where I originally had the idle set.
    Do you think I should go up 2.5 or 5 on the pilot jets?

    My current jetting is: 130 mains, 115 / 110 / 112.5 pilots (bow to stern)



    Mike

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