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  1. #1
    CB750 New Member vintagesportbikes's Avatar
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    Hesitation when bliping the throttle

    My 1981 750f is hesitating when I run the throttle up. It idles good at 1000rpm, but lags as I hit about 3500-4000 rpm, then after a second or two revs up quickly. I cleaned the carbs (they didn't look very dirty) and installed new jets, float needle valves and main needle and pilot valves. The pilot valves are set to appx 3 turns out. The main jets are #100. The carb intake boots are brand new. the air filter is used, but was installed at the time. Any ideas are appreciated.

  2. #2
    Moderator dirtdigger's Avatar
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    Probably in the accelerator pump circuit but you will never get rid of all of it. How does it run when you are riding the bike under a load? No load blipping or revving of the engine really has no point or relevance to how the engine actually runs. Best thing you can do is put on a set of Mikuni RS or Keihin CR carbs.

  3. #3
    CB750 New Member vintagesportbikes's Avatar
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    Quote Originally Posted by dirtdigger View Post
    Probably in the accelerator pump circuit but you will never get rid of all of it. How does it run when you are riding the bike under a load? No load blipping or revving of the engine really has no point or relevance to how the engine actually runs. Best thing you can do is put on a set of Mikuni RS or Keihin CR carbs.
    I haven't ridden it yet. The bike was not running (or apparently stopping) when I got it. The hesitation I see when twisting the throttle is abnormal, but I can't tell if it's flooding or starving. I think it would bog down under load until it caught up a second or 2 later. . Admittedly, when I stripped the carbs, I did not disassemble the accelerator pump because I had no rebuild kit on hand but I guess I'll pull it apart and see what I find. Thanks.

  4. #4
    Moderator dirtdigger's Avatar
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    The accel pump is critical. These carbs run lean for emission and rely heavily on the accel pump.

  5. #5
    CB750 Addict pidjones's Avatar
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    And cleaning the accelerator nozzles is a pain. I used a high G from a 12 string guitar set chucked in a pin vise to reach in and open the plugged holes on mine, then blew liberal amounts of carb cleaner through them, then exercised the (new) pump and circuit with isopropyl alcohol to be sure. If you buy an ebay kit, beware that the Taiwan vendor ships by slow boat. Better to pay extra and by a North America based kit.
    "Love 'em all.... Let GOD sort 'em out!"

  6. #6
    CB750 New Member vintagesportbikes's Avatar
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    Quote Originally Posted by pidjones View Post
    And cleaning the accelerator nozzles is a pain. I used a high G from a 12 string guitar set chucked in a pin vise to reach in and open the plugged holes on mine, then blew liberal amounts of carb cleaner through them, then exercised the (new) pump and circuit with isopropyl alcohol to be sure. If you buy an ebay kit, beware that the Taiwan vendor ships by slow boat. Better to pay extra and by a North America based kit.
    I appreciate the comments. Good advice. Thanks guys.

  7. #7
    CB750 Member NicksHonda's Avatar
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    I have the same issue. Mine will idle fine. It hesitates all around under 4K whether blipping the throttle or under load. After 4K it takes off and seems to run great above 4K.

    I sprayed WD40 all around the intake boots and no noticeable change in RPM.
    I have the same cylinder pressure on all cylinders.

    I noticed on mine, my #2 cylinder is running much cooler than the other cylinders. After it runs for a while, if I spray a little fluid on the pipes, the other three burn the water right off and sizzle. #2 doesn't sizzle the water much.
    My accelerator pump seems to dump a lot of fuel in, this is on #2 of course.
    Good luck.
    Havin' Fun in Sacramento!

    2006 BMW R1200RT
    2002 Honda CBR954RR
    1979 Honda CB750L

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