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Thread: Killing During Idle After Carb Rebuild

  1. #21
    Supporting Member Yendor's Avatar
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    If you increase the Float Adjustment distance you wil LOWER the fuel level in the float bowl and make it harder for the jets to pull fuel up thru the jets & tubes to the intake throat.

    I suspect that is your problem already your system is starving for fuel.
    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

  2. #22
    Junior Member nest711's Avatar
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    Quote Originally Posted by mike in idaho View Post
    "The butterfly on my left carb is worn to the point where it cannot get tight enough during bench sync to match the right."
    Carb butterflies don't wear out, if the butterfly is not a good fit in the carb body someone has had it apart and put it back together crooked, loosen the screws and re-align it so it can meter fuel/air properly.
    "The right spark plug is still fouling, but new issue is the left plug doesn't seem to be used at all. It looks brand new (put a new one in after changing the floats)."
    Pull off the
    air cleaners and squirt a small amount of gas or carb cleaner into the carb throat while it's running. If it picks up a little bit doing that it's too lean, if it stumbles it's too rich.
    What does the battery voltage read, after it quits? Long periods of idling is a guaranteed way to kill the battery, it doesn't put out enough watts to keep up with the current draw under 3,000 rpm.

    Let me rephrase what I said earlier: the carb under the influence of the middle screw between the two has it's spring completely compressed and still cannot match the tightness of the other carb (the one under the influence of the idle screw) for a bench sync.

    If I am understanding you correctly you want me to unscrew the two screws holding the butterfly in place, let it drop down and then realign it so that it is even within the frame. If I do this will I be able to match the tightness of the two carbs without the middle screw being completely compressed?


    Quote Originally Posted by Yendor View Post
    If you increase the Float Adjustment distance you wil LOWER the fuel level in the float bowl and make it harder for the jets to pull fuel up thru the jets & tubes to the intake throat.

    I suspect that is your problem already your system is starving for fuel.
    So you think I am running too lean now or that I was always running too lean?

    You're prescription would be to take the carbs apart again and re-measure the floats to the stock 18.5mm at an angle and proceed from there?



    Also I found several problems today:

    1. The carbs have started leaking fuel out of the breather tubes.

    2. The fuel line has started to leak so I think I need tighter fitting line. Got some proper 5.5mm coming in the mail as we speak.

    3. After taking the carbs apart again I noticed that the float bowl gaskets were not holding in place, so I have ordered new ones that will fit the shape without having to finagle them in and have them warp.

    4. Battery wise the one I had been sold was bad so I exchanged that out at the store today for a fresh one, so that is no longer a factor either.
    Last edited by nest711; 04-14-2019 at 06:46 PM.

  3. #23
    Supporting Member Yendor's Avatar
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    If the butterfly plates have been removed and replaced by anyone other than you, they may have been installed backwards.
    The plates sit closed at a slight angle, and have a BEVEL on the edge so they close tightly.
    Make sure the bevel side is insatalled so it closes.

    While you have the caarbs apart IF they are leaking out the overflow tubes check them for cracks.
    The Overflow tubes frequtnely develope stress cracks BUT... are easliy repaired by soldering them.

    Float heights are critical for the JETS to function correctly.
    Too high and they will pull fuel up thru the jets into the intakes too freely amd cause rich running.
    Too Low and the vacum will not be able to pull fuel up thru the jets. causing a lean condition.
    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

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  5. #24
    Junior Member nest711's Avatar
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    I'll be able to check the tubes after work today, but since I'm grounded until Friday (when the gaskets and new fuel line come in) I want to try and clear some stuff up. Basically I'm confused as to how the bike could be running lean if the plug is fouling. The blackness of the plug is not shiny so I'm sure it's not oil fouling. Could it perhaps have been a combination of all the other issues, i.e. the leaks in the fuel line and float bowl gasket, that was causing it to go back and forth between too lean and too rich?

    Basically I'm back at the drawing board until I get these parts in as those could have been influencing the problem easily. New plan is to re-clean and put the carbs back together setting the floats to the stock 18.5mm (measured at an angle) and seeing what happens from there. I'll probably get new coils too just to future proof that and hopefully fix the left side spark.

    Speaking of which does anyone know if the 4into1 coils are any good? A single common motor coil is as expensive as two 4into1 coils, so if there's not much difference I'd rather save the money.

    Update: the tubes are all good no cracks at all.

    Also I finally broke down and called a local mechanic that specializes in Japanese bikes and he had some really helpful insights that I'd like to relay to you guys and get your opinions on. Basically he said that by idling for more than five minutes I'm trying to tune/jet the carbs from a complete dead cold to near overheating (since the bikes are air cooled and not supposed to idle for extended periods of time). As long as it's idling for at least five minutes I should be good. His advice was to install the new gaskets, fuel line, ignition coils, and reset the floats to stock value, Afterwards I should go ahead with a carb sync, put the tank on, and finally go for a ride and see how it goes. He even offered to put it on the dyno and give me the readings to see how the fuel to air ratio looked after I put it all back together.

    I'll be sure to keep you guys posted as things unfold over the couple weeks; I'm eager to get this baby on the rode at spring starts to come into full swing.
    Last edited by nest711; 04-20-2019 at 05:46 PM.

  6. #25
    Junior Member nest711's Avatar
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    Update: I figured out why the left plug wasn't fouled: the points cover was shorting out the left point and preventing the left side from firing. I also went ahead and installed the new ignition coils, float bowl gaskets, and 5.5mm fuel line. The bike starts up right on first kick, but there's still some issues: it's absolutely impossible to vacuum sync the carbs. The right carb refuses to go over 10cmHg while the left one will happily move within spec (from 16-24cmHg). I was told this could be occurring because of a weak cylinder? Despite this I was able to ride the bike around the neighborhood for around 45min, but I did notice that the bike really wants to kill on turns/when it's not getting consistent throttle input. For reference both plugs are now black as well.

    At this point I'll probably get a gauge and do a compression check, but other than that I'm sort of at a loss at where to go next. The Honda service manual says that if a carb is below 15cmHg to check Ignition timing (which I have and it is pretty damn spot on), tappet clearance (which I still need to do), spark plug gap (spot on perfect), and compression pressure (which I'll be testing within the next week). At this point it looks like I may end up having to get an engine rebuild to get the bike running better, but admittedly I'm torn as to whether or not it's worth it. Any input from you guys would be appreciated and thanks for all the help throughout the last few weeks- you guys have really made it possible for me to get to this point and I'm really thankful for that.
    Last edited by nest711; 04-20-2019 at 09:28 PM.

  7. #26
    Sensei 66Sprint's Avatar
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    Quote Originally Posted by nest711 View Post
    Update: I figured out why the left plug wasn't fouled: the points cover was shorting out the left point and preventing the left side from firing. IF this was the case, it should have been fuel fouled (wet)....IF it was actually getting fuel....

    For reference both plugs are now black as well. Are you running with the choke on?
    See Blue comments above......
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."

  8. #27
    Junior Member nest711's Avatar
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    Quote Originally Posted by 66Sprint View Post
    IF this was the case, it should have been fuel fouled (wet)....IF it was actually getting fuel....
    The left plug was never carbon fouled. When I did pull it out of the hole to check there was a strong smell of fuel, but just to iterate it looked like it came out of the box brand new.

    Quote Originally Posted by 66Sprint View Post
    Are you running with the choke on?
    The choke lever is in the down position, so the butterflies are open and the choke is off. It doesn't need need any choke to start (it'll go on the first kick no problems). The fact that both plugs are fouled do lead me to believe it is still running rich. I could always raise the float value again as after installing all the new equipment I reset the floats to the stock 18.5mm.

    As a side note I got a rental compression tester kit from the auto store today and will be testing tomorrow morning I'll edit this post with an update when I have the values.
    Last edited by nest711; Yesterday at 12:36 AM.

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