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  1. #1
    Moderator OsideBill's Avatar
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    Carburetor Drilled Return Pressure Balancing Modification GPR, XLT, GP, SUV, etc.

    I get asked all the time about this modification so I decided to make a "how to" and make it a sticky.

    This is one of the simplest modifications to do to your carbs. It is intended for multiple carb applications, but can be done on a single carb application as well.

    The goal of this mod is tie triple carbs with individual fuel pumps, into a single balanced pressurized system. Rather than three individual systems

    One advantage to this modification has been, reduced fuel pump check valve failure. This is a common problem on the older style SBN carbs, used on the GP1200, SUV and many other skis. This failure can lead to potential lean condition, and piston failure. I ran a set of SBN's with this modification in my SUV, and I had over 300 hours on the carbs. When I inspected them recently, all the check valves were in pristine condition. Pretty much unheard of for these style carbs.
    When I would get a set of SBN carbs in for repair, I could guarantee the cutomer I would find a creased check valve, and I was right 99.9% of the time.

    Another reason for this mod, is the small return orifice in the carb body, gets plugged easily. Either by a foreign object, or corrosion. The orifice is .020" in diameter. This condition results in the fuel pressure getting to high within the carb, which in turn forces the needle valve to open and floods the carb.

    There are several ways to do the restrictor either with a jet installed in an inline fitting (Tee or coupler) or threaded into the tee on the carbs.

    Novi carbs come with the returns drilled out, I will attach their schematic.

    On Yamaha 1100 carbs with the remote fuel pump, the carbs were set up in this manner. They had a restrictor built into the last tee on early models. Later models they went to a small orifice in each cover.


  2. #2
    Moderator OsideBill's Avatar
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    The Return orifice shown on SBN and SBN-I (GPR) carbs
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by OsideBill; 07-21-2012 at 01:22 AM.

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  4. #3
    Moderator OsideBill's Avatar
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    Drill the return orifice out, using an 1/8" drill bit. It is not very deep, stop when the hole breaks through into the fuel outlet cavity. The fuel outlet cavity is where the nipple for the return line is either threaded or pressed into the carb body.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  6. #4
    Moderator OsideBill's Avatar
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    Tap the last tee on the #1 carb where the line returns to tank with an M5 tap, the hole is already the correct size no need to drill.
    For GPR's and XLT's, Install a 95 Mikuni Main Jet
    For older GP's and SUV's use a 105 Mikuni Main Jet. 97-99 GP1200's have a smaller fuel line so it is necessary to either change the fittings or use an inline restrictor.

    By insatlling the restrictor Jet in this fashion the Mod always stays with the carbs
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  8. #5
    Moderator OsideBill's Avatar
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    Novi Schematic
    Note: They show a #60 jet in the return restrictor this has been changed to 105-110 per Tim at Novi.
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Name:	Novi Fuel line routing.jpg 
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  10. #6
    Moderator OsideBill's Avatar
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    Older style 1100 carbs, return to tank nipple on carbs
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Name:	Chris 1100 020.jpg 
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  11. #7
    Predator-x's Avatar
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    I´m a little confused or maybe i´m not reading well , but is drilling needed for a GPR or can i only use the inline restrictor ?

  12. #8
    Moderator OsideBill's Avatar
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    no need for the inline restrictor unless you drill.

  13. #9
    Predator-x's Avatar
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    Thanks ,now it makes sence ,i thought that that a inline restrictor replaces the drilling mod ,ill go for drilling with a 95 main jet.

  14. #10
    Predator-x's Avatar
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    One last question to be sure before i start drilling, do i need to drill each orifice hole on each carb ?

    Thanks,

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