cb450 high idle
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  1. #1
    Senior Member MechaTomato's Avatar
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    cb450 high idle

    I did some searching around this thread and was unable to find anything, so sorry if there's already a thread with this answer

    Anyway, the problem is that I cannot get the idle to go below 1500 rpm. I had just recently rebuilt the carbs, with 150 main and 36 (i think) pilots, and had synced both carbs using guitar string as the measurement. I also replaced the carb boots and gaskets, and replaced the felt seals on the throttle shaft and applied grease to it.

    When I first put the carbs on after the sync with the guitar string, It would idle at 2000 rpm cold. As it warmed up, the idle would get higher. I could lower the idle screw (evenly on both sides) but the rpm would only drop to 1500 before the butterfly valves would just completely close, with no more room to lower it.

    Any thoughts on this?'

    edit: I read something about the timer advance springs would be worn and wouldn't close all the way. Took a look on mine and the springs were not pulling the assembly back together the way it should . I cut the end of the spring and bent the hole to make it shorter. It works now. I redid the timing after fixing this.

    It helped with the idling issue, but did not solve it. It brought it down from 2k to 1.5k
    Last edited by MechaTomato; 03-31-2019 at 06:51 PM.

  2. #2
    Supporting Member David Babbitt's Avatar
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    Tell us more about the guitar string calibration. Never heard of it.

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    Junior Member Marcus17's Avatar
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    Try the "straw test" on your carbs. If the slides don't return it will keep the RPM's up. I couldn't get the idle down on my bike until I polished and cleaned the slides and covers. They should move up and down freely without sticking. https://www.hondatwins.net/forums/56...traw-test.html

    cb450 high idle-img_20181026_133141.jpg

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  5. #4
    Senior Member MechaTomato's Avatar
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    Quote Originally Posted by David Babbitt View Post
    Tell us more about the guitar string calibration. Never heard of it.
    I saw a video on youtube about it. you put a guitar string between the wall and the butterfly flap and turn out the idle screw until the string pulls out. You match the resistance n both sides and that should "sync" the carbs. It's an easy way without having to add vacuum taps on the intake.

  6. #5
    Senior Member MechaTomato's Avatar
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    Quote Originally Posted by Marcus17 View Post
    Try the "straw test" on your carbs. If the slides don't return it will keep the RPM's up. I couldn't get the idle down on my bike until I polished and cleaned the slides and covers. They should move up and down freely without sticking. https://www.hondatwins.net/forums/56...traw-test.html

    Click image for larger version. 

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    Just tried it. Blowing into the two top holes resulted in the slide moving. I was unable to get it all the way up (probably because of how much larger the diameter of the straw was than the hole) but it was moving smoothly and would always set back down into position. I could flick it up with my fingers and it would come back down fine

    The brass hole on the bottom had no effect on both carbs however. Blowing into that did absolutely nothing to the slide, even with a lot of air.

  7. #6
    Senior Member HondaPete89's Avatar
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    Read your FSM. Learn how your carburetor works. You want to focus on the relationship between idle stop screws, and air/fuel mix screws.
    Marcus17 likes this.
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  8. #7
    Senior Member MechaTomato's Avatar
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    Quote Originally Posted by HondaPete89 View Post
    Read your FSM. Learn how your carburetor works. You want to focus on the relationship between idle stop screws, and air/fuel mix screws.
    I understand how the carb works for the most part. The problem is that I can't lower the idle. The butterfly valve in the carb completely closes but is still idling at 1500 rpm. I'm unable to adjust anything if I can't get the idle lower

  9. #8
    Junior Member Marcus17's Avatar
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    Moving up and down easily is a good sign. With these carbs you want to 1st check for any air leaks, check the slides and make sure nothing is clogged. Did you put the carbs in a ultrsonic cleaner and check that all passages were clear? Here is a diagram of the carbs, the upper holes create the vaccuum to move the slide (I'll confirm this on my bike tomorrow).

    cb450 high idle-cb450-carb-diagram.jpg

  10. #9
    Junior Member Marcus17's Avatar
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    Here is a reference file for the CB450 with the initial setting for the carbs.

    https://docs.google.com/spreadsheets...it?usp=sharing

  11. #10
    Sensei 66Sprint's Avatar
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    No, the two holes in the bottom of the slide piston on the engine side allow vacuum to affect (reduce slightly) the (previously atmospheric) pressure up above the slide......Full Atmospheric pressure entering through the "vacuum port" gets under the slide and lifts it until the pressure differential equalizes..... This is why the slide does not lift much at small throttle openings......
    However, as you open the throttle, the differential is more as the volume and speed of the intake air (across the slide holes in the venturi) increases the vacuum above the slide causing it to rise higher to its balance point.....
    Marcus17 and workdogz2 like this.
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."

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