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carb breather line routing

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by idontlikemondays, Sep 22, 2007.

  1. well, this is something which has caused me a bit of grief lately. i'd really appreciate abit of a technical explanation as to why inncorrect routing adversally affects your bikes performance. when the bike stutters at cruise is it because its running lean, or running rich?
    tweet, in regards to my post a while ago that you helped out on, this was the issue! serves me right for paying off the manual!
    any thoughts/expertise imparted, as always would be greatly appreciated!


  2. Carb breather hose routing is important. Sorry if this gets a bit basic:

    A CV (vacuum) carb operates by the difference between:
    - normal air pressure through the breather hose, and
    - the 'vacuum' (reduced air pressure) produced by the engine turning over.

    A slide is attached to a rubber diaphragm, one side of the rubber has normal air pressure, the other side has the engine suction. The rubber diaphragm is attached to an air valve (slide) and fuel needle/jet.

    The engine suction is increased or reduced by you turning the throttle valve, which blocks the engine's fuel/air intake.

    As you change the throttle position and thus the engine's suction, the diaphragm gets different vacuum pressures and the slide+needle gets pushed up (more air/fuel) or down (less air/fuel).

    IF you route the breather hose incorrectly it could get kinked, or the end of it could face into the wind or out of the wind, affecting the air pressure on the OTHER side of the diaphgram, and the amount of fuel and air going to the engine.

    Clear as mud?
  3. ahhhh, so incorrect routing affects to operation of the slide. but the vaccum must be acting on the diaphragm end of the slide (to open it when more fuel is needed) and the breather line is inbetween the daiphragm and and the slide. so if the breather line is kinked it will stop the slide (and needle) from opening fully, hence causing it to run (way) too lean?
    but hang on a second, that doesnt make sense, as that would mean acceleration would be affected, not cruising. or have i got the orientation and the point of effect on the slide arsed about???

    so what regulates the position of the slide? throttle position or RPM? or both??

    thanks heaps for the reply hotcam!
  4. i did look this stuff up on howstuffworks.com, but they only talk about a basic carb setup, without a vaccum chamber setup.
  5. OK: The engine suction varies by RPM only.

    It is reduced by a butterfly valve between the carb and the engine, which is attached to your throttle. We'll call this 'regulated suction'.

    The slide is completely independent, it goes up and down by normal air pressure on the bottom of the diaphgram VS 'regulated suction' on the top of the diaphgram.

    So the slide position is indirectly controlled by a combination of engine RPM and throttle position.

    The point of this setup is so that by opening the throttle at low RPM you don't get a huge amount of fuel/air into the engine more than it can handle. It also smooths the rate of opening the throttle (compensating for hamfisted insensitive sudden throttle use).

    Kinked breather line might cause lean running or weak running. Less noticeable under acceleration because you've got the vacuum less regulated then.

    Also in a RAM-AIR bike the breather operates at RAM pressure, not normal air pressure, and should lead to the ram air intakes.

    You could also have a fuel starvation issue, check your fuel filter and hoses to make sure they're not blocked/kinked and your carb float level and needle/seat operation. But first check your carb breather.
  6. thanks again hotcam!
    the point which i dont understand is that if the point of vaccum comes from the venturi, how is the slide retracted (sucked) away from the venturi??
    i have the carbs apart on my bike a few times, im just trying to understand how the slide is regulated?
    thanks again mate, i really do appreciate your help!

  7. The venturi vacuum is piped to the top of the diaphgram and the atmospheric pressure is piped to the bottom of the diaphgram. There should not ideally be any vacuum escaping up the slide to the bottom of the diaphgram.
  8. ahhh now i understand perfectly. hence why a restricted bike such a hyo 650 has holes in the carb slide, to stop it from completly opening. does make me wonder though, when i put the jet kit in the firestorm, a had to drill an extra hole into the slide? maybe thats why its top end is waaayyyy shitter than it should be?
    thanks heaps for you persistance of explaining this to numptie like me hotcam!! :grin: