Stalling CB175 at idle
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  1. #1
    Senior Member lhemrick's Avatar
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    Stalling CB175 at idle

    I have a 1972 CB175 that has been completely rebuilt from top to bottom. I have set the carbs by screwing in the idle screws till it starts missing and turn them back out just it runs good agian. It wasn't doing this at first but recently when it is idling for lack of a better word it will cough very slightly back through the carbs and quit. You can crank it right back up and it will idle for a short time and do it again. When you ride it it runs just fine but when you let off the gas to start stopping it will do it again. Any ideas anyone?
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  2. #2
    Senior Member middletons's Avatar
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    Timing and valve adjustment spot on? Float levels set correct? Point gap set? Mech advance in good working order? Throttle cables and slides work freely?
    1972 CL175
    1972 CB350

  3. #3
    Senior Member lhemrick's Avatar
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    Quote Originally Posted by middletons View Post
    Timing and valve adjustment spot on? Float levels set correct? Point gap set? Mech advance in good working order? Throttle cables and slides work freely?
    Yes to all but I will go back and double check them all.

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  5. #4
    8AD
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    it happened to me, then I fixed two main easy things before going for other more complex one:

    setting the carb float level
    the main jet clogged a bit, so I cleaned it and cleaned the its thread hole, it was full with carbon/gasoline residue.

    just a little bit of my experience.

  6. #5
    Senior Member Simo's Avatar
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    Are the carbs synced?
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  7. #6
    Junior Member mtn_'s Avatar
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    Are both carbs "coughing" or just one? This would really help eliminate a lot of things as it's likely both carbs didn't develop the same issue at the same time and the problem could lie internally. An easy test is to run the bike on one cylinder at a time, also helping you diagnose the problem. If you're sure its the carbs as i suspect, a full rebuild is the only way to truly find all carb problems. this doesn't always mean you need to buy new pieces, the only ones i usually replace are the rubber gaskets. everything else can clean up well fully disassembled. some people bead blast the bowl chamber, a cheaper way is to use a dremel carefully cleaning afterwards. main concern would be the gas float pin under the bowls when disassembled. these really like to stick, i lightly sand the sides and top seat of the pin. you will see a residue line.

    the other possibility is a timing issue. if it sparks while the intake valve is open your ill get the cough back through the carbs i believe. any out of time spark should be a real concern to the motor. if the ignition timing test proves true, your cam chain could be loose and the timing will actually change as the bike runs instead of static. this can often be corrected by the front cam chain tensioner adjustment bolt, then a full re timing of the ignition with the contact points and F mark on the alternator. you should also check your valve clearances in proper time with the T mark on the alternator. if any intake valve stays open slightly the cough could result. your contact points could also be arcing if the timing is correct. this happens when dirty and can cause spark issues and timing issues.

    hope this helps!
    mtn_

    1968 CL175 K0 project (long-term perfection)
    1969 CL175 K3 daily
    1970 CB175 K4 daily
    1972 CL175 K6 project (bore)
    1972 CB350 K4 project (frame)
    1969 CT70 K0 project (fuel/spark)

  8. #7
    Senior Member lhemrick's Avatar
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    I've already addressed most of the issue's you suggested, timing, valves and a rebuild of the carbs making sure all passages are open. Now when I was checking my float heights I found them a little out of adjustment and one of the float pins was bent. I replaced it with a spare and at least that stopped the overflow problem I was having. I also replaced all the gaskets with OEM Honda ones. That leads me to another question? With the floats set to 21mm and the carbs at an angle on the bike would the fuel level be over the gasket on the engine side? I have a little leakage from the right carb from around the float bowl. I started it today and can make it miss screwing the air screw in to rich side but can turn it out all the way out with little effect. When I get it set where it runs its best it won't start to save its life when its cold. I'm embarrassed to say I realized one thing today that causing an air leak is the cable sealing caps are missing off the top of the carbs. That's got to be a small leak but could that be causing my problem?

  9. #8
    Senior Member Simo's Avatar
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    Yes any air leak is going to mess with your tuning
    Here's Steve's how to to set carbs the air screw and slide stops are set as a baseline to start not an absolute
    I start with the slides synchronized (cable adjustment), throttle stops backed out, and the mixture screws turned to the center of their specified range......
    Hold the throttle slightly open and start the engine..... Back the throttle down until you get an idle ~1200 RPM, HOLD it there, and gently turn in the throttle stops until it starts to rev.....adjust back to 1200.... Once it is again idling at 1200 with both throttle stops set, you are ready to do the actual adjustments......

    Pick a carb....adjust its MIXTURE screw throughout its spec'd range to find the point of highest idle RPM, and leave it there at highest revs...adjust that carb's throttle stop back down to 1200 RPM...
    Same procedure, other carb......
    Repeat on both carbs to verify/fine tune.......Repeat again IF necessary....
    Re-verify slide sync...Adjust IF necessary (usually no more than 1/4 turn on the carb top adjuster, often they are still fine)...
    If you did this correctly, vacuum , exhaust pressure, and exhaust heat will be the same on both sides (hard to verify last two on the CL)
    Engine response to throttle should be "crisp" and smooth throughout entire range....
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  10. #9
    Senior Member lhemrick's Avatar
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    Thanks for that info Simo! Question is what are my carbs mixture screws specified range? I've seen charts on this site with ranges depending on what type of carb. I don't know what I have because these didn't come with the bike and I see no numbers on them like all my 350 twin carbs do. Besides if I'm not running stock air cleaners and exhaust that range would differ anyway correct?

  11. #10
    Senior Member Simo's Avatar
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    What size jets? Also look for a 3 digit number printed on the manifold flangesStalling CB175 at idle-cl175-jetting2.jpg
    Last edited by Simo; 01-01-2017 at 08:37 PM.
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