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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 Enthusiast 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
    1979 Honda CB750L

  8. #8
    CB750 Enthusiast NicksHonda's Avatar
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    My issue was #2 wasnít running in the lower rpms. It flooded out below 4K. The air cut off circuit will make it run rich when the diaphragm has a tear. Most people apparently plug them as they only help with popping on hard deceleration.

    Good luck with yours.

  9. #9
    CB750 Guru amc49's Avatar
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    NO, the air cut diaphragm having a tear LEANS the engine out. TWO idle airbleeds there and one gets cut off with a working diaphragm to go richer, torn it does not ever shut off. As well, the leak also then feeds the cut vacuum actuation port into carb bore and runs even leaner due to that as it becomes a vacuum leak.

    The Euro spec DOHCs have no accelerator pumps on them at all and run fine. The biggest issue the DOHC has with accel pumps is the check valve stuck due to ethanol damage to carb bowl there on #2 carb.

    #2 carb may have idle passages clogged, that makes the engine go super rich past idle due to the wonky seesaw effect the idle and off idle works on these, what normally is lean is rich and there are reasons for it. The bikes are known for it and why everybody keeps tearing into the carbs as well as over the engine being down on compression due to nobody ever setting the valves right. That makes the slides not lift as much and the engine goes rich due to that too.

  10. #10
    CB750 Enthusiast NicksHonda's Avatar
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    That's interesting... my #2 plug was wet with fuel. I plugged the ACV (side port) and now it runs well. I assumed it was running too rich to fire the #2. Maybe it was the accelerator pump. IDK.
    Havin' Fun in Sacramento!

    2006 BMW R1200RT
    1979 Honda CB750L

  11. #11
    CB750 Enthusiast NicksHonda's Avatar
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    Havin' Fun in Sacramento!

    2006 BMW R1200RT
    1979 Honda CB750L

  12. #12
    CB750 Guru amc49's Avatar
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    That guy is most certainly an expert but following his stuff does not guarantee perfect running if not all of it is done at the SAME TIME. These carbs throw most people for loops who until they ran into them thought they were pretty good at carbs, these show you otherwise.

    They however can be made to run for long periods of time with zero issues but the first thing you must have is a 100% rock solid engine as the carbs will not run at 100% without that. CV carbs rely on the engine draw to open fully, and these are the worst as they are solid slide operated, they do not respond as quick as normal CV slides with rubber diaphragms.

    Nobody ever sets the valves because they require shims and then the engines just slack off in performance when the valves begin to burn and most people then blame the carbs, which can be an issue but lots of times are not. Bike then sits and THEN the carbs become a major issue.

    The primary jet circuit is positively linked to the idle or pilot circuit on these and the primary relies on the idle circuit to lean out a certain extent to make the primary jetting correct, when the idle fuel feed clogs the IDLE leans out but as soon as you transition from idle to PRIMARY flow the flow goes RICH as the plugged up idle restriction kills the idle leaning out the primary, what I mean about one circuit seesawing the other, they both work at the others expense. The engine typically is super lean at idle then goes super rich just above idle, why everybody goes nuts trying to figure out what the h-ll is going on.

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