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Thread: synchronizing carbs

  1. #1
    Registered User
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    synchronizing carbs

    Good Day , I am synching the Bings on my 1980 R100RT.
    Using my new 'TwinMax' balancer
    all is going well, I went carefully through the Snobum procedure and I have the engine idling nicely at around 900 it sounds sweet and the gauge is spot on zero even when I turn up the sensitivity .
    Okay here is the bit I DONT like.
    When I go under the carbs to set the idle mix the RIGHT hand carb behaves perfectly, starting from one turn out from a soft closed position I slowly wind the screw in until the revs start to drop off , note the position and then wind the screw out again the revs pick up and then start to drop off again , so I set the screw mid way between these two points. Fine .
    NOW on the left hand carb when I try to do the same thing I am finding that the revs increase as I wind the screw in!
    not much now but the fastest idle certainly seems to be when the screw is set right home.
    I watched a video tutorial of Chris Harris synching the carbs on a 75/6 and he had exactly the same issue
    he did not go into the problem but said he was not happy with the carb.
    Now, I re-built these carbs with Bing kits so it is very likely that I have put something in upside down or back to front.
    Plagued by self doubt , anyone have any helpful comments, very much obliged
    regards Heed

  2. #2
    MOA #24991 Pauls1150's Avatar
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    Easy things first: verify that both pilot jets are the same size and are properly oriented & seated, and that all O-rings are in good shape & not cracked or distorted.
    If closing in the air screw all the way increases the RPM, that strongly implies that there's an air leak somewhere between the pilot circuit and the intake valve. Likely culprits would be the intake manifold plumbing & the big O-ring (try a liberal spray of WD-40 to see if it causes any change); possibly a crack in the carb casting (need a bright flashlight & a magnifying glass).

  3. #3
    Registered User kentuvman's Avatar
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    Wondering how you "cleaned" the carbs - I use an ultrasonic cleaner - the jets can be blocked - suggest checking O rings, as in Post above and re-cleaning jets and finishing job with compressed air - a strand of copper wire through the jet passages should help too.

    Let us know how it goes.
    Ken Tuvman
    Excelsior, MN
    K75s, R65LS, R60/5

  4. #4
    Pistol Shooter headhog's Avatar
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    I believe what you have is a carburetor that is running rich at idle. When you turn in the idle mix screw you shut off that fuel supply and the leaner mixture causes the idle to increase. The other sources of fuel are the enricher circuit or a butterfly that is not closing properly.

    I would check to make sure the enricher, for the problem side, is completely off. Then check to see if the enricher was installed properly. The enrichers are marked for the left or right side and can be mixed up during a rebuild. Also the enricher shaft can be oriented upside down and this can be checked by the alignment dot. Snowbum describes both of these situations and how to identify them.

    If this fails to turn up the problem I would next make sure the butterfly is closing completely. Again during a rebuild, the butterfly can be installed backwards or not positioned properly. (This is something Snowbum also discusses.) Either case will allow air leakage and fuel to be drawn from the main jet at idle.

    Once you find the problem you will need to re-synchronize the carburetors.
    MSF Instructor
    1994 R100R Mystic
    2007 Harley Electra Glide Classic
    1979 Harley FLH

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