1992 750sx 44 SBN mikuni jetting with factory pipe
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  1. #1
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    1992 750sx 44 SBN mikuni jetting with factory pipe

    Hello all,
    I have a 1992 750sx. All stock engine. Has factory limited pipe. Compression is 155 on fresh rebuild. The boat runs fine except from 0-30% throttle. Not as bad if I roll throttle on slowly but if I stab it it basically hesitates for a few seconds and then goes. Ive searched many threads on baseline jetting for my setup and 150 high/120 low seems to be ball park. Iíve tried changing low speed jet to 125,127.5 with no difference, Iíve tried 120 and 115 without much change either. Plugs always seem to be on rich side with dark color. Iím looking at the suggested jetting that factory pipe recommends on the stock Keihin carb which is the following

    750SX
    Main jet : 155
    Pilot Jet : 88
    High speed screw : 7/8 turn out from closed Low speed screw : 5/8 turn out from closed Needle & Seat : Stock
    Spring : Stock

    While I realize the mikuni and keihin are 2 totally different carbs the low speed jet seems to be a lot smaller than what Iím using. Can anyone offer any insight on how the keihin to mikuni jetting cross references? I looked up a chart online and it seems to suggest a 88 Keihin correlates with a 82.5 mikuni. So Iím wondering if I should start trying smaller low speed jets and see if this will help. Maybe start at 100 and work my way down? Any insight will be appreciated! Thanks in advance

  2. #2
    Top Dog JC-SuperJet's Avatar
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    Did you do an Air Leakdown Test on this Engine before sending it?

    If you have any Air Leaks, it will be near impossible to tune either the Mikunis or the Keihins.

    Do you have a Single Keihin Carb set-up? Stock Jetting is #75 Pilot and a #140 or #145 Main Jets.

    So Factory Jetting specs for Keihin make sense. How do you come up with a #120+ Low Jet? Again, are you using a Keihin or a Mikuni Carb? What size exactly? 38mm, 40mm, 44mm? Be specific.

    Aside from finding any Air Leaks, I would rebuild the Carburetor INCLUDING THE PUMP (Keihin).

    Make sure the Pulse Fitting and Pulse Hose are good and tight.

  3. #3
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    I’m sorry. . I’m running a SBN 44 mikuni. I put that in the thread title but forgot to do it in the actual post.
    Last edited by Thorny; 12-02-2021 at 03:56 PM.

  4. #4
    Top Dog JC-SuperJet's Avatar
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    This Jetting is what should work with your setup:

    Kawi 750SX with A/M Factory Ltd Pipe and SINGLE Mikuni 44mm Carb & Flame Arrestor

    Main Jet = #150 with High Speed Adj @ +1-1/4 turns

    Pilot Jet = #120 with Low Speed Adj @ +1-1/2 turns

    1.5N/S x 80gr Spring = 38psi Pop-Off

    Skat-Trak Impeller 9/17 or 10/18

  5. #5
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    Ok thanks. I’m running that exact jetting but I’m running 2.5 N/S with 80 gram spring. I have a 1.5 N/S. I’ll try it. I’m running a Solas 11/16 Concorde for prop.
    Last edited by Thorny; 12-02-2021 at 04:17 PM.

  6. #6
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    Quote Originally Posted by JC-SuperJet View Post
    This Jetting is what should work with your setup:

    Kawi 750SX with A/M Factory Ltd Pipe and SINGLE Mikuni 44mm Carb & Flame Arrestor

    Main Jet = #150 with High Speed Adj @ +1-1/4 turns

    Pilot Jet = #120 with Low Speed Adj @ +1-1/2 turns

    1.5N/S x 80gr Spring = 38psi Pop-Off

    Skat-Trak Impeller 9/17 or 10/18
    So by reducing the Needle and Seat size to 1.5 It will raise popoff and lean fuel mixture correct?

  7. #7
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    Quote Originally Posted by Thorny View Post
    So by reducing the Needle and Seat size to 1.5 It will raise popoff and lean fuel mixture correct?
    Raising the pop-off will make the idle and pilot mixture a lot leaner. You can set the idle mixture out of the water by closing the bottom adjuster until the engine races then opening it until it slows back down. Make sure to do this any time the pop-off is altered. The pop-off pressure will determine how far out the bottom screw must be opened. The pilot jet is in play around 1/4 throttle or when the ski is about to plane. Listen to the exhaust, you want it to be sputtering a little. Too lean will sound very clean and too rich will be sputtering a lot. Pilot jets are sensitive so one size change will make a noticeable difference at this throttle position. The main jet and top screw are plumbed together and control the fuel coming out of the main discharge nozzle. You want to open the top screw 3 turns out and see how it accelerates at full throttle. Slowly close it until the ski pulls hardest. If you cant make it run worse by opening the top screw then install a larger main jet and try again. Ideally you want to find best acceleration with the top screw around 1 turn out. Your 11-16 concord is very low pitched, this makes card tuning easy.
    Last edited by kweasel; 12-02-2021 at 07:25 PM.
    What exactly does the IJSBA do?

  8. #8
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    Quote Originally Posted by kweasel View Post
    Your 11-16 concord is very low pitched, this makes card tuning easy.
    Thank you sir! Appreciated. Thanks for taking the time to explain. Iíll pop that 1.5 n/s in and see how she goes. The boat runs very strong once itís cleaned out and on plane. It only radared at 45 mph though so I figured I was under propped. It gets there in a hurry though. I ditched my SXR 1500 and wanted a vintage boat like I had in the 90ís. Been out of the 750 game for a long time so I am relearning all this.

  9. #9
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    Here is problem I am having. You can see me stab throttle and lean forward over the handlepole and it takes a few seconds for the boat to take off. Not sure if you can hear motor stumbling.

    Last edited by Thorny; 12-02-2021 at 09:04 PM.

  10. #10
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    Quote Originally Posted by Thorny View Post
    Thank you sir! Appreciated. Thanks for taking the time to explain. Iíll pop that 1.5 n/s in and see how she goes. The boat runs very strong once itís cleaned out and on plane. It only radared at 45 mph though so I figured I was under propped. It gets there in a hurry though. I ditched my SXR 1500 and wanted a vintage boat like I had in the 90ís. Been out of the 750 game for a long time so I am relearning all this.
    It may be slightly under propped if you have the original unported cylinder. I wouldn't worry about it though unless I was racing or holding it wide open for a long time. The old 750s were hard to turn at top speed anyway. I know the next size up concord 12-18 has a lot more load and would likely need to be pitched down a little.
    What exactly does the IJSBA do?

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