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  1. #1
    CB750 Member PRIMEWORKS's Avatar
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    Mar 2016
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    Winnipeg
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    25

    High idle at operating temp when clutch is engaged?

    so I'm in the process of test riding my 1981 cb750k every other day because I've just recently completed a rebuild on it and have run into a bit of a running issue and I wanted to run it by some of you. So to break down my current set up, I have 4-1 aftermarket MAC exhaust with a baffle in it, I've bench tuned my carbs, I'm running pods, have large jets (98's & 125's), and have the mixture screw at about 1.75 - 2 turns out. I've noticed that when the bike gets to operating temperature a couple odd things tend to happen:

    1. When decelerating I often times here cracks and pops from the exhaust which to me implies that I still have to much fuel entering the combustion chamber and not being ignited. I've heard that this could potentially be bad?

    2. When I pull in the clutch to shift gears my RPM spikes slightly between shifts and when I pull in the clutch when I'm coming to a stop my RPM rises and stays high as long as the clutch is in. When I'm at a stop i'll adjust my idle knob on the carbs to lower the idle and the RPM drops but then the bike basically comes close to dying/does die because the rpm is to low at that point.

    The odd thing is that when I first test drove my bike with this set up it ran like an absolute dream! I'm yet to have a chance to do a full inspection myself just yet (check spark plugs or a potential air leak) but i figured i'd post and see if someone might think it's something else/ know exactly what this might be.

    Build thread in case anyone is curious : http://www.dotheton.com/forum/index.php?topic=73590.0

  2. #2
    CB750 Addict Dave's Avatar
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    Sep 2012
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    Ca
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    497
    #1a Pods. Though it may run good enough for you, It'll never run as good as it did with the stock airbox.

    #1b My bike has the stock air box and it "cracks and pops" occasionally under deceleration (Using the engine to slow the bike) too. It's fairly common in these older bikes, especially ones running modified exhausts with lower backpressure.

    #2a Is the clutch cable somehow physically moving the throttle cables? Like do you have them zip tied together?

    #2b Adjust the idle when the bike is at full operating temperature. Use the throttle to manually keep it from dying when it is cold. This is the normal operating procedure for this model.

    #2c Are you saying your bike won't idle (run at the proper idle rpm) when warm? This is an entirely different problem.
    96% of the time I edit my replies so check back for updates if it isn't at least an hour old

  3. #3
    CB750 Addict Wez_'s Avatar
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    Apr 2017
    Location
    Enfield, CT
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    283
    I had the same problem. It was a combination my push pull throttle cables not being adjusted correctly and a proper carburetor synchronization with the gauge and adapters. Bench sync is ball park to get the bike running. You can never adjust those throttle plates accurately on the bench. You will think it has to do with the clutch, but if you're using the clutch, your also using the throttle to scoot around. While opening and closing those 4 throttle plates in the carb, on a NON sync'd carb bank, you will likely have extra fuel or too little air or many other wrong air/fuel combinations going on (potentially x3 for carbs 1, 3 and 4). Temperature and humidity will also slightly effect things but not majorly. Do the slack adjustment on the throttle cable towards the back of the motor. The front one just gets tightened. The back one can slightly be keeping the throttle plates opened. There should be slight slack in the cable before the plates are engaged. At the carb, there are two nuts that work together to place the inner cable and outer sheathing in the proper spot for the rear most throttle cable. There is also a cable adjustment at the throttle handle on the cable sheathing to further help get the cable/sheathing correct. Once the cables are squared away, synchronize the carbs with the gauge and adapters. Tighten everything and you should be good. If you slightly let off the clutch enough to engage the engine, but not enough to move, do the RPMS drop to normal when you are experiencing the higher RPMS? That's how my bike behaved before the fixes.

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