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  1. #1
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    Carb sync

    I have set my carbs as Clymers suggested. Then I adjusted my valves to their spec. Then I connected carb synchronizer. I use the kind with mercury in tubes and the idea is to equalize the height of the mercury in two tubes. Do I do this at idle or some other RPM?
    Campbell Tellman II
    1993 R100RT

  2. #2
    Administrator 20774's Avatar
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    Both. At idle, you will be setting the idle mixture and the idle speed. You will need to do one, then the other, and go back to the first as they interact with each other. Then you need to set the throttle cable tension...do this by running the RPM up to 1500-2000 and adjusting the tension in the cables. Generally, you want to take the carb which has the higher speed (vacuum) and lower it by decreasing the tension in the cable. The reason is that if you increase the tension in the slower carb, you run the risk of reducing the slack that you initially started with. That messes up the idle settings.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    #4869 DennisDarrow's Avatar
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    For me, and you will get varied responses to this subject with each having their own merits, I use the values achieved with mercury sticks or even the el cheapo $4 guage to check out idle air screw adjustment, idle speed, and then up where I cruise or get full advance on the timing curve. There are reasons for using them at each place; but I find that they are a great double check for the idle speed and stuff; but what really matters is getting the engine running smoooooooth at the RPM where I ride.
    NO I do not spend a lot of time making the adjustment at 3K RPM; but we are only talking about perhaps tightening or loosening one of the cables by 1/8 of a turn at the most to get it right.

    ALWAYS..........take a ride to warm up the engine before you make these adjustments.......NOT around the block but 10 to 20 miles.........

    Use the largest floor fan you can get in front of the engine while making the adjustments. The whole thing should take more than perhaps 5 minutes; but that short bit of time can really put a lot of heat stress on one's engine.........

    God bless.........Dennis

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    Bing Instructions . . .

    I have a set of instructions from Bing. It will allow me to do a proper synchronization. Thanks for your help!
    Campbell Tellman II
    1993 R100RT

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    Mercury

    Need any mercury? I think I have some flowing around my shop, it's probably trying to find a lower point.......get that thing going, let's ride somewhere!

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    A Source for Mercury

    After I spilled the synchronizer and lost most of the mercury I went to my Dentist and he said "I have some and don't use it anymore." Ergo he gave me a big container to refill the synchronizer!
    Campbell Tellman II
    '93 R100RT

  7. #7
    Benchwrenching PGlaves's Avatar
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    Quote Originally Posted by CTellman View Post
    I have set my carbs as Clymers suggested. Then I adjusted my valves to their spec. Then I connected carb synchronizer. I use the kind with mercury in tubes and the idea is to equalize the height of the mercury in two tubes. Do I do this at idle or some other RPM?
    Campbell Tellman II
    1993 R100RT
    Long answer. Caveat: folks do it different ways. Here is how I do it and why.

    Engine warmed up, box fan blowing from the front.

    1. Adjust the cables so there is slight slack at each adjusting ferrule. I like about 1mm slack.

    2. Adjust the idle speed screws to equalize the vacuum on the two sides.

    3. Loosen the locknuts on the cable adjusters. Turn the throttle until it is just off idle and on the cables. Adjust the cable adjusters until the vacuum is equal and you still have slight slack in both cables when returned to idle.

    Now the why. You are attempting to set the throttle plate angles the same. You are using vacuum as a surrogate for airflow which is itself a surrogate for throttle plate angle. At the lowest RPM while on the cables, the cross section for air is a very small crescent, and any slight angular difference in the throttle plates will be a significant percentage of air flow, and vacuum. At larger throttle openings slight angle differences don't result in as significant vacuum changes. You get maximum sensitivity and accuracy at the smallest throttle openings.

    I also know that some folks have engines that seem out of synchronization at 4,000 RPM if synchronized the way I do it. This is not because of throttle plate angle however. It is because of other factors that interfere with air flow. Deposits on valve backs and stems, deposits in the intake tract, carbon in the cylinders, and valve adjustment are some of these factors.

    So I set my carbs to synchronize throttle plate angle as closely as possible at low RPM. Then, If synchronization drifts significantly off at higher RPM I look for and address the root cause which is disturbing air flow.

    Synchronization at road speed RPM is a good get-me-home strategy for a poorly running engine but that is the only time I would personally do it.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://www.bigbend.net/users/glaves

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