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Thread: Air bypass screws - what are they really for?

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    Question Air bypass screws - what are they really for?

    Ref; 1999 R1100S

    As you know there is some great info on how to properly sync TB's from guys like Bob Lentini and others. All of the info I've been able to find just tells you to open the air bypass screws a number of turns (somewhere between 1 to 3 turns is what I've seen) and start from there. But, and here's where I might be completely off base, my other ride is a 1988 Ducati Paso 750 and it is equipped with a Weber DCNF 44 (2 barrel carb). Getting a carb that's designed for a car on a bike is a real trick but that's not my point. Aside from the air/fuel mixture screws there are two air bypass screws, one for each barrel. The idea on the carb is to only adjust one so that the airflow equals the other barrel. Thus my question for the TB's on the R1100S.

    I've been thinking about better low end throttle response. Has anyone ever started the sync process (assuming the TPS is properly set) by completely closing the air bypass screws, properly setting the idle RPM's / sync on both TB's by adjusting the throttle cables at the butterflies and then using the air bypass screws for fine adjustments?

    If I understand the fuel injection system properly, it will automatically compensate for mixture. Doesn't opening the air bypass screws a substantial amount create an enhanced possibility for an overly lean mixture? It would seem to me if you wanted a more accurate injection mixture then you wouldn't want to introduce extraneous airflow. Especially that far downstream where the only sensor that can compensate for it is in the catalytic?
    Current Rides;
    1999 R1100S
    1988 Ducati Paso 750

  2. #2
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    The bypass screws control a relatively small amount of air compared to throttle plates. Don't see how trying to use them in reverse wil improve either synch accuracy at any speed or throttle response.
    OTOH, an AF-XiED is transformative on Motronics bikes...

  3. #3
    Benchwrenching PGlaves's Avatar
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    Quote Originally Posted by ahdoman View Post
    Ref; 1999 R1100S

    I've been thinking about better low end throttle response. Has anyone ever started the sync process (assuming the TPS is properly set) by completely closing the air bypass screws, properly setting the idle RPM's / sync on both TB's by adjusting the throttle cables at the butterflies and then using the air bypass screws for fine adjustments?
    This ignores the fact that throttle plate angle at idle is set by the "do not touch" idle stop screws with the blue paint - not by cable adjustment. It also would seem in fact to not allow "fine adjustments" because if you start with the bypass screws closed you can only adjust one way - out - whereas if you start with the bypass screws as specified you can make adjustments in both directions, in or out.
    Paul Glaves - "Big Bend", Texas U.S.A
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    IBA #44567 Ken F's Avatar
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    OH~ OH! A swing and a hit......and it's going, going, gone! Outa the park folks!
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  5. #5
    Registered User roger 04 rt's Avatar
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    Quote Originally Posted by PGlaves View Post
    This ignores the fact that throttle plate angle at idle is set by the "do not touch" idle stop screws with the blue paint - not by cable adjustment. It also would seem in fact to not allow "fine adjustments" because if you start with the bypass screws closed you can only adjust one way - out - whereas if you start with the bypass screws as specified you can make adjustments in both directions, in or out.
    +1

    Stepping back from this interesting question for a moment, the fuel injected boxer engines are really two engines sharing a common crankshaft, using with a common O2 sensor, and a single throttle opening sensor on the left-hand cylinder. To further complicate matters, it is run lean to meet emission needs which means that drivability suffers whenever there is a power difference at light loads. In other words, unless the airflow is identical in each cylinder, and the amount of fuel injected is identical, the motor isn't its best.

    (The R1200 is a big improvement because there are two O2 sensors (which get the fueling balanced), and two idle air adjustments controlled by its ECU (which compensate for intake leakage differences and air volume needed at different air temperatures.)

    So to get to the OP's question, assuming you have perfectly matched injectors, the problem becomes keeping the airflow matched at all throttle angles. Below is a chart showing how sensitive the engine's output power is to throttle angle at light loads. For this reason, I would not attempt to flow balance the throttle bodies myself (but that's just me) and would leave the paint alone on the throttle bodies and TPS. However, BMW has provided an adjustment to compensate left/right closed-throttle air balance caused by differing leakages: The idle-air-bypass screws, whose purpose is to balance left/right air leakage at small throttle angles.

    I don't mean to be a heretic, but because Rob Lentini was a great experimenter but working in the blind (no measurement tools for left/right AFR and no BMW documentation on the Motronic), and not here to defend his work, I will simply say that I see Zero=zero, altered coding plugs and GS intakes tubes on RTs (and the converse, RT tubes on GSs) to all be ineffective ideas.

    Keep the throttle bodies clean, rebuild when the shafts are worn, keep the throttle cables adjusted correctly, use the coding plug designed for the motor, and use the idle-air-bypass screws for the intended purpose of insuring airflow balance at small throttle angles.
    RB

    Last edited by roger 04 rt; 01-22-2014 at 04:21 PM.

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