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  1. #1
    CB750 Member Hunter1919's Avatar
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    Hanging idle with disengaged clutch

    Just got this bike running after 5 years of sitting. Cleaned and synced carbs and put new clutch cable in plus other unrelated things. It idles fine before riding. When I come to a stop itll hang at 3k rpms with the clutch pulled in. If i let the clutch out itll drop back down to 1k. Throttle isnt stuck and i believe i did a good job on the carbs. Is this a clutch issue?

  2. #2
    CB750 Guru amc49's Avatar
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    Throttle sticking or a vacuum leak. Or not all carbs coming completely shut if the 4 cable type.

  3. #3
    CB750 Member Hunter1919's Avatar
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    It's a k8 so 2 cables. It has new boots on airbox and engine side. Where else could it leak from? My first thought was the throttle but when I have it in the garage it's working fine. It has a nice snap back.

  4. #4
    CB750 Guru amc49's Avatar
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    The only way you can hang idle is the engine MUST be getting air/fuel to do it. Ignition advance stuck can possibly do it.

    Throttle can snap back all day long, that does not mean it is dropping back to full 100% zero, only to where it stops possibly from sticking. It can stick for a bit at so little above 100% off you can't tell by hand at all. Could depend on if your sync job was mechanically done or by vacuum. You could possibly yank the rack to detail if all four bores have the slides just covering the first hole at bottom of bore that appears when slide begins to open up. If holes showing fully the idle is set too far open. Correct is no hole or the slightest bit showing and it exposes the instant throttle begins to open. All 4 need to be dead even too. One being off can do it.

  5. #5
    CB750 Member Hunter1919's Avatar
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    I'm pretty sure the slides sat right when I reinstalled the carbs but I'll double check. I'm learning as I go so I could've easily overlooked something. I synced it with a set of vacuum gauges and adjusting the nut on the top of the slides. So I'm not sure if that's mechanically or by vacuum.

  6. #6
    Moderator dirtdigger's Avatar
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    Check the ignition advancer. These are very prone to getting sticky. They can be at base timing at idle on startup then when you get the rpms up its sticks till you shut it off or put some sort of load on the engine that brings the rpms down and it allows the advancer to work its way back to base timing. Can be either rust or old sticky lube in the advancer, advance weight pivots could be worn, springs could be getting weak. Or if installed incorrect the part that moves on the advancer could get wedged when the nut on the end is tightened down. Things get bent on that end too because people put a big wrench on the big nut and crank the motor over, this easily bends the small stud that holds the advancer on.

  7. #7
    CB750 Guru amc49's Avatar
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    X2 that and he would know as he is 'the' SOHC guy here.

    Sorry for the confusion on syncing. They ALL set mechanically as that is the only physical way you can do it. What I meant was by using like very small drill bits say 1/32" in all 4 bores, carefully done it will sync mechanically as well as any other method can, the slides will move dead evenly.

    However, as engine gets older you may not want or need that as the individual cylinders will begin to wear in different ways and then sometimes you want say one slide to be up a bit higher to begin with or lift at motion a bit faster in an effort to make them all closer together. Done by vacuum only then, a pure mechanical method will not work there. In effect you are mis-syncing slightly to bring the 4 cylinders in closer together in realworld action due to wear influences. Needed say for one cylinder not seating as well at the valves to then need to lift fractionally quicker to stay up with others, an example. I have a CB550 that requires that, it has a lot of wear and time on it being an all original bike from '77, but play with it long enough and it gets to where it pulls off idle really well, but not easy to get there. The slides actually get somewhat uneven there but the bike pulls off idle very well like that. If you set them all dead even (mechanically, not by vacuum) it pulls away doing all sorts of funny things, you can hear the different cylinders fighting with each other as one comes on later or earlier than the others. That does not happen on new engines though, they run fine set like that.

    Since carb bank is off engine at rebuild time I tend to mechanically preset the slides and then watch what the engine wants once it carboncoats to begin to run like it will longterm. If error shows up then I set by vacuum.

  8. #8
    CB750 Member Hunter1919's Avatar
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    Thanks yall. I'll check the advancer and if that isnt it I'll pull the carbs again.

  9. #9
    CB750 Member KRBJ's Avatar
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    Have you set up your fast idle cam and choke?

  10. #10
    CB750 Member Hunter1919's Avatar
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    So it was the carbs. The slides on 1 and 4 were sticking open. The bike is finally fully functional.

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