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  1. #1
    CB750 New Member Timberfield's Avatar
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    93 CB 750 Nighthawk idle issues

    Recently got this bike, only has 4000 miles. Went over the carbs twice, and still having issues with idling. Runs on all four no problem with the choke on. Soon as I take the choke off it runs like junk. When riding with the choke off, it runs great at half throttle or better. Major hesitation and miss firing and lower RPM. My thought is starving for fuel. But I have checked everything, new plugs, new battery. Cleaned the fuel tank and petcock. Does anyone have any ideas? Any help would be appreciated.

  2. #2
    CB750 Addict pidjones's Avatar
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    Clean the carbs again (yes, again) using the SeanG manual.
    "Love 'em all.... Let GOD sort 'em out!"

  3. #3
    CB750 New Member Timberfield's Avatar
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    Question about slow speed jet

    Quote Originally Posted by pidjones View Post
    Clean the carbs again (yes, again) using the SeanG manual.
    So I am in the process of rebuilding following the SeanG manual. I removed the slow speed jets and assume cleaning the jets themselves is sufficient (no other small passages below the jet hole to clean with e-string. Please see the attached pic.

    My question is regarding the rubber passage plugs that cover the slow speed jet hole. I don't have them?? I suppose its possible my rebuild kit didn't have them, and I didn't replace on my first go around (maybe threw the old ones out)?? Can you confirm if this type carb has the rubber plugs? It seems the SeanG manual covers several types of carbs.

    My bike is a 93 Nighthawk, and my carbs do not have an accelerator pump.

    Click image for larger version. 

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  4. #4
    CB750 Addict pidjones's Avatar
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    https://parts.southernhonda.com/imag...MW34E1801C.jpg
    Doesn't look like the '93 CB750 has those plugs.
    "Love 'em all.... Let GOD sort 'em out!"

  5. #5
    CB750 New Member Timberfield's Avatar
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    Much better, but have a lean pop

    Quote Originally Posted by pidjones View Post
    https://parts.southernhonda.com/imag...MW34E1801C.jpg
    Doesn't look like the '93 CB750 has those plugs.
    The carbs are back together and back on the bike. It runs WAY better now, idle is great, power and response are good.

    I am noticing that I have a lean pop at low speed coming from the left side. I'm assuming its the pilot screw adjustment. When I disassembled the carbs, I took note that they were all about 1 1/2 turns out. The manual said 2 5/8, so I did 2 5/8, but it seems pretty far out and don't think they could go much more without falling out.

    Any thoughts on this? I really appreciate your help!

  6. #6
    CB750 New Member Bill S's Avatar
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    I had same problem with 92' and found carbon around butterflies. Use a good carb cleaner on that area after taking carbs off bike. My bike ran fine at upper rpms but had to slip clutch and a lot of gas off idle before cleaning now runs right off idle. My bike has 26K on it. I had to use choke too when I stopped at a light before.

  7. #7
    CB750 Guru amc49's Avatar
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    The mixture screw needs to be where a HONDA brand service manual tells you where to put it. Out more on that type is richer. Problem being those screws and the seats in carb body damage super easy as soon as somebody runs them in too hard to deform the seat. Once that happens you cannot simply count the turns any longer as the seat deformation blew all that out the window, the restriction point there is damaged and you have to guess at the turns on that one as they may be wildly off what the other carbs are. One way to tell damage is to lightly seat all the screws with carb rack off and look at how much of the screw tip presents itself at the bottom of the carb bore, if all are close to even you are OK, but if one comes out much more than the others you got damage there.

    If you hold carb with the front facing you then any teeny little holes that are just covered up with the throttle butterfly fully closed MUST be open and the small chamber that supplies them with fuel as well, it is blocked off with a plug under carb behind the mixture screw hole. Those holes act like accelerator pumps as the throttle gets opened past idle and clogged the low rpm performance will suffer greatly.

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