Killing During Idle After Carb Rebuild
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Thread: Killing During Idle After Carb Rebuild

  1. #1
    Junior Member nest711's Avatar
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    Killing During Idle After Carb Rebuild

    This is probably a redundant set of questions/concerns as I think I know what the issue is, but nonetheless it's better to be safe than sorry.

    Context: Just finished rebuilding and bench syncing my carbs two or so weeks ago, but it's been raining and I've been busy so they haven't been able to get on. My impatience got the better of me today and I decided to put them back on and fire the bike up under the carport despite pouring rain since I'm eager to see how she runs after a whole tune-up and sitting for a season. Much to my pleasure she fires right up first kick at half-choke, but I quickly have to go and lower the throttle-stop screw to keep the revs from going crazy at idle. From here all is dandy as she's warming up for five or so minutes but as time goes on I notice the revs start to slow down until it kills completely. I play with the throttle-stop screw and get her back up and going but I cannot seem to find a mid point: either the throttle screw is in too much leading to excessive idle rpm or it is too low and gradually sinks down from 2000rpm to zero.

    I know the carbs still have to be vacuum synced but shouldn't the bike at least idle long enough to get sufficiently warm enough to actually sync them? I was thinking that perhaps the bike was being fuel starved since I had the gas tank off the bike and connected to the carbs with extra long hoses that may have been preventing adequate fuel delivery. I also noticed that it would sometimes begin to kill after grabbing hold of the throttle without twisting (like it wasn't getting enough fuel or something).

    Should any of this concern me; do I just set the throttle screw high enough so it idles without killing to get warm enough to then proceed with a vacuum sync, or is this possibly a larger issue that will preventing the sync from even being achievable. Thanks for any insight guys.

    Bike is a CL360 '75 by the way
    Last edited by nest711; 03-10-2019 at 09:22 PM.

  2. #2
    Senior Member mike in idaho's Avatar
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    I would check for an air leak first. Spray some WD40 or carb cleaner around the mounting to see if it affects the way it runs.
    '65 YG1
    '65 CB160
    '66 CL160
    '67 CL77
    '68 TR6
    '69 T100R
    '69 T120R
    '72 Commando 750
    '78 XS650E
    '79 Gl1000
    '81 440 LTD
    My company car is a Kenworth

  3. #3
    Junior Member nest711's Avatar
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    Quote Originally Posted by mike in idaho View Post
    I would check for an air leak first. Spray some WD40 or carb cleaner around the mounting to see if it affects the way it runs.
    I'll go ahead and give that a try on Tuesday as I'll be too busy at work all day tomorrow to try. I have my doubts that an air leak is the problem considering I have brand new intake manifolds with new gaskets as well as NOS Honda air filters. I maybe could have had a leak at the exhaust because they were mounted in a hurried fashion but once again I have my doubts.

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    Junior Member nest711's Avatar
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    Given the information I've found in this post https://www.hondatwins.net/forums/49...ver-tubes.html I've decided to check the plugs as soon as I can for any possible fouling. His bike's symptoms sound eerily similar to what I am experiencing, but before jumping the gun I need to properly set the mixture screws and vacuum sync the bike. If I'm lucky that will fix the issue all by itself.

    For reference I set my floats to 19.00mm as an in between from Common Motor's suggestion and Motor Mayhem's suggestion, » Honda CB Carburetor Rebuild &raquo Motor Mayhem .

    In the meantime any further advice is appreciated, but I will be updating this post as soon as I find the time to work on the bike.
    Last edited by nest711; 03-10-2019 at 09:38 PM.

  6. #5
    Supporting Member Yendor's Avatar
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    I would make a guess that you have set the float wrong.

    What model Carbs do you have?
    19mm seems small.

    There are couple of things to consider here.

    1st is the inverse relationship to float hiehgt and fuel level in the bowl.
    The smaller the gap in the float hiehgt the higher the level of fuel in the bowl.
    This is because the float hangs upside down. As you set the gap smaller the float rises and so does the fuel level.

    2nd is th TIP of the Needle that rests on the Float Tanfg is spring loaded.
    If you are not holding the carb at the 45 deg angle as specified in the manula you will compress that spring tip and have a bad setting.
    Again this typically will result in a fuel level that is too high and will cause flooding condition and one that the idle mixture screw seems to have no effect on.
    Crashoverride likes this.
    1970 CB 350 CAFE - Current Project on the bench,
    1972 CB 350 K4 Red - Now a Happy Rider ! !
    1972 CB 350 K4 Green (My Sons) DONE - YES !,
    1st Bike 1970 SL 350 (Brought home in Parts - Trailer/Trunk/Back Seat - I miss that bike),
    2nd - Bike Kawasaki 750
    3rd - '73 XLCH 1000

  7. #6
    Senior Member mike in idaho's Avatar
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    If you suspect the float's too high, turn the petcock off after you start it, see if that makes a change.
    '65 YG1
    '65 CB160
    '66 CL160
    '67 CL77
    '68 TR6
    '69 T100R
    '69 T120R
    '72 Commando 750
    '78 XS650E
    '79 Gl1000
    '81 440 LTD
    My company car is a Kenworth

  8. #7
    Junior Member nest711's Avatar
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    Quote Originally Posted by Yendor View Post
    I would make a guess that you have set the float wrong.

    What model Carbs do you have?
    19mm seems small.
    I have the stock Khelin carbs for a '75 CL360, which the manual states to have at 18.5mm. So that we're not shooting around in the dark I'll be checking for any fouling/gas smell on the plugs tonight after work. If any is present I'll go ahead and raise the floats a bit to around 20-22mm.

    Quote Originally Posted by mike in idaho View Post
    If you suspect the float's too high, turn the petcock off after you start it, see if that makes a change.
    I would be doing that after it warms up and begins to lower in rpm, correct?

  9. #8
    Junior Member nest711's Avatar
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    The plugs are black as can be and smell slightly of gas, so it seems my suspicions have been confirmed. Will be adjusting float level tomorrow several mm higher and let you guys know what's up.

  10. #9
    Supporting Member Yendor's Avatar
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    My thought on the Fuel Level being too high was the Spring Loaded Tip on The Needle Seat.
    The Needle that contacts the Float TANG is Spring Loaded.
    If you don't hold the Carb Body at an angle as prescribed in the FSM, the weight of the the float can compress the spring giving you a false reading that is too large.
    The end result is to high a fuel level.
    Crashoverride likes this.
    1970 CB 350 CAFE - Current Project on the bench,
    1972 CB 350 K4 Red - Now a Happy Rider ! !
    1972 CB 350 K4 Green (My Sons) DONE - YES !,
    1st Bike 1970 SL 350 (Brought home in Parts - Trailer/Trunk/Back Seat - I miss that bike),
    2nd - Bike Kawasaki 750
    3rd - '73 XLCH 1000

  11. #10
    Junior Member nest711's Avatar
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    Quote Originally Posted by Yendor View Post
    My thought on the Fuel Level being too high was the Spring Loaded Tip on The Needle Seat.
    The Needle that contacts the Float TANG is Spring Loaded.
    If you don't hold the Carb Body at an angle as prescribed in the FSM, the weight of the the float can compress the spring giving you a false reading that is too large.
    The end result is to high a fuel level.
    Whenever I adjust them this time around I'll be extra careful to hold them at the proper angle and not accidentally compress the spring whenever measuring them. I'll also measure them before bending to ensure that I am actually setting them to a different, higher value, than what they are now. Higher value equals less fuel in the bowl correct?
    Last edited by nest711; 03-11-2019 at 04:19 PM.

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