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  1. #1
    CB750 Member 1slowCB's Avatar
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    Pops on decel & hanging rpm

    I was having a hanging rpm problem before and noticed that the intake boots were cracked. I replaced them. Carburetors have been cleaned. After replacing the intske boots, i assumed that the carbs weren't in sync. I tried synching them and just couldnt get it to work. I believe it was a vacuum leak somewhere because when I am riding at any speed over 30kph, the revs start climing if I pull the clutch in. I have to set the idle adjustment screw all the way back to avoid this, but then it wont idle (obviously because idles set too low). It also has a pops on decel.

    Where should I go from here? Where should I check for vacuum leaks, as I sprayed the intake and carb boots and got no change in rpm?
    This things been driving me insane.
    Using FACTORY AIRBOX.

  2. #2
    CB750 Guru amc49's Avatar
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    If you have messed up sync to where you have one or more carbs very far open at the idle screw backed way off you will still get hanging idle. The main adjusting screw means nothing if the carbs themselves are open. You must get that sync issue right.

    If the carbs have never had the aircut valves looked at there can be vacuum leaks there too. They stop popping in pipes at decel.

  3. #3
    CB750 Member 1slowCB's Avatar
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    I bench synced them before i put them in and still had the issue.

    I will check the aircut valves. Any other sources of vacuum leaks?

  4. #4
    CB750 Guru amc49's Avatar
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    You have to break the rack apart to check aircuts.

    No other normal places for leaks but being humans work on them anything is possible. If the idle passages are still clogged then they become in essence a vacuum leak. Have seen carbs cleaned 3X and still be clogged. Saying it on these does not mean doing it.

  5. #5
    CB750 Member 1slowCB's Avatar
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    Damn. I was hoping to not have to break the rack apart. Thanks for the help!

  6. #6
    CB750 Guru amc49's Avatar
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    They break into pairs at the middle with no trouble but to go further you have to remove at least one choke plate on each pair, they use staked screws and they ruin when you yank them, also often ruining the choke shafts. The only way to recoup those parts is to carefully remove the staked portion of the screws on the backside where they stick out, then the screws can usually be reused. I use a dremel with a rock in it to do that work in minutes.

    Draw a pic of the way the choke springs assemble in the middle of the bank or you will never get them back together right. Or............

    http://www.cb750c.com/publicdocs/Sea...anual_revG.pdf

    Page 7

  7. #7
    CB750 Member 1slowCB's Avatar
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    Thank you! I will check it out.

  8. #8
    CB750 Member 1slowCB's Avatar
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    Quote Originally Posted by amc49 View Post
    They break into pairs at the middle with no trouble but to go further you have to remove at least one choke plate on each pair, they use staked screws and they ruin when you yank them, also often ruining the choke shafts. The only way to recoup those parts is to carefully remove the staked portion of the screws on the backside where they stick out, then the screws can usually be reused. I use a dremel with a rock in it to do that work in minutes.

    Draw a pic of the way the choke springs assemble in the middle of the bank or you will never get them back together right. Or............

    http://www.cb750c.com/publicdocs/Sea...anual_revG.pdf

    Page 7
    So I bypassed the air cutoff valves and checked the valve clearances and they were between 0.003 - 0.005. Im still getting the hanging rpm problem, unless I completely turn the idle knob all the way out. Also bench synced the carbs again. I still get some popping on decel, but its reduced now.

  9. #9
    CB750 Member 1slowCB's Avatar
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    Anything else I should look at ?

  10. #10
    CB750 Guru amc49's Avatar
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    I got nothing left, you have it all and never had problems once I did those things. Says to me something you did not done right.

    Sticking throttle or cable? All you got left.

    Hanging idle is leaking air or throttle held open in some way, no other way to get it.

  11. #11
    CB750 Member 1slowCB's Avatar
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    Brand new motion pro cables. Lubed them as well.

    I think you're right. I'll have to double check all my work.

  12. #12
    CB750 Guru amc49's Avatar
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    If you have not adjusted some built-in slack into the cables them being new means quite literally nothing.

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