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Thread: 1977 R100S Bing Idle / pilot jet

  1. #1
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    1977 R100S Bing Idle / pilot jet

    So.. chasing a gremlin somewhere in the bike--
    Bike's hard to start, but once started, it starts bucking and loses power at around 3000-4000 rpms... cutting out and stalling with accompanying backfiring...
    Checked points, cleaned carbs, fuel flow & bowl level, valves.... so far everything looks good...

    This came on suddenly last week- she was running fine and then started losing power mid-throttle... naturally I thought either bad gas or crud in the bowl(s)...
    Nope on either...

    When she gets warmed up, she runs.. almost okay- still a slight stumble at 3K-4K rpms, but very slight. And when you back off the throttle, you can she's thinking about stalling out....

    So I'm going to go back to the beginning in adjusting the carbs, but one question I can't find in the bing manual regards the Pilot Jet. Does it get backed out any amount after returning it to its hole? I was thinking it did, but can't find any reference to the amount if so.... (And just to clarify: I'm talking about the Pilot jet, not the mixture screw)

    PS - another quick question-- did we lose a lot of content from the forums when they were switched over? I seem to recall oodles of info that I can't seem to find now by searching....?

    And once again, thanks in advance!

    Bob

  2. #2
    Registered User lkchris's Avatar
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    At 3000-4000 rpm it's all main jet and you likely have a torn diaphragm. Recommended change interval for diaphragms is yearly, although I hasten to add nobody does that. Just double check, too, that both needles are in the same notch.
    Kent Christensen
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  3. #3
    MOA #24991 Pauls1150's Avatar
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    Agreed, but yearly?? (ymmv...)
    The pilot jet is FIXED in position. Also make sure the "stack" of jets is aligned and per the Bing picture, and that the needle is free to slide thru them. I'll try to PM you with the diagram, but it's a large PDF so may not make it -?

    Could also be an issue with the coils and/or plug wires.

    EDIT - nope, couldn't send the file, so try here:
    http://www.bingcarburetor.com/pdf/MO...DED%20VIEW.pdf

  4. #4
    Bluenoser
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    When something happens quickly after a bike has been running fine and you believe it is fuel related then it can be anything from the fuel delivery end to plugging off in one of the passages in the carb. Make sure you have good fuel delivery when you turn the petcocks on ( just hook a plain clear hose to each), put the other end into a clear container and turn them each on to check for a good flow. Another thing that does happen ands causes all kinds of weird running symptoms is any water in the fuel system.

    Backfires are usually an indication of a lean condition. Colour of your header pipes and how far down the blue colour goes also help in indicating a lean condition.

    All the jets in the carbs are screwed in tight. Assembly order with the main jet is important as you can install one of the brass atomizing pieces in up side down. Don't get jets mixed up with the adjustable mixture & idle screws. These screws are adjustable, jets are not ( except that you can put in a larger or smaller diameter jet for certain altitude conditions.

    You have to go back to square one with the carbs, take them apart, check everything, blow air through all the passages and make sure it exits, when done with that set the adjustable screws at the Bing spec, adjust all the cables ( choke & throttle ) so that you have about 2 mm of slack at each cable where it goes into the adjuster on the carb. ( This will let you sync the carbs properly once you get the idle set.

    Working with carbs is a bit of an art form, it can be learned and if you are not familiar with them, find a local air head owner who can help you.
    1971 R50/5 SWB with R75/6 drivetrain
    2013 DL650

  5. #5
    Administrator 20774's Avatar
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    Quote Originally Posted by bobinthemtns View Post
    PS - another quick question-- did we lose a lot of content from the forums when they were switched over? I seem to recall oodles of info that I can't seem to find now by searching....?
    Not lost...just hard to find. The search tool, or better yet, the index of searchable items is not really working correctly. But using google's "site" search works pretty well. Read more here:

    http://www.bmwmoa.org/forum/showthre...-the-MOA-forum

    As to the stalling issue at speed...you mentioned fuel flow...was that sitting in the driveway? What about cracking the gas cap the next time this happens on the road...could be a vacuum is building up in the tank if it's not venting properly.
    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!

  6. #6
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    Thanks for the feedback guys--

    I had already checked the diaphragms, no tears, they looked fine.....
    And, like I said, it just happened suddenly, so I doubt it was a natural/gradual degradation of of the diaphragms.... but their replacement is on the list...

    Cycleman2, I too had always heard backfiring was a symptom of a lean condition, but the Bing Manual says backfiring in the exhaust pipe (where mine's happening) is actually from a too rich condition... I'll check the spark plug to see if it's wet or dry tonight...

    And I thought the pilot jet was fixed flush with its hole.. but was hoping that maybe that was my issue...

    Yeah, swapping out the coil wires/plugs etc is next.. but I have pulled the plug wires off while the engine was running to make sure each cylinder was getting power.. and they were... but perhaps it's a weak spark....

    And thank for the link, Kurt- yeah, seems the new search function is a bit more limited- that'll help! Oh, and yeah, cracking the gas cap was one of the first things I did....

    Alright, well I'll be back at after work today!

  7. #7
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    backfiring

    I, too, have understood over the years, by any carb mfg. that backfiring is due to too rich a condition as all the extra (from being too rich) fuel is not getting burned up in the cylinder, but is "spilling" out into the inside of the exhaust pipes where the hot pipes ignite the raw fuel causing the backfiring sound.

    Check the plugs, too.

    I once had a Honda that the plugs just "went" one day after sitting a while. I tried everything. Nothing made sense as I did all the tuning, set timing, valves, etc.

    Bought a new set of plugs, and viola, backfiring instantly ceased!

    Also, check the plug wires for any sort of shorts, wear, etc. which might cause the plugs to fire intermittently.



    Go figure!

  8. #8
    Registered User m_stock10506's Avatar
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    Quote Originally Posted by bobinthemtns View Post
    Thanks for the feedback guys--

    I had already checked the diaphragms, no tears, they looked fine.....
    And, like I said, it just happened suddenly, so I doubt it was a natural/gradual degradation of of the diaphragms.... but their replacement is on the list...

    Yeah, swapping out the coil wires/plugs etc is next.. but I have pulled the plug wires off while the engine was running to make sure each cylinder was getting power.. and they were... but perhaps it's a weak spark....

    Alright, well I'll be back at after work today!
    Coils or plug wires could be your problem. But be aware that pulling the plug wires off while running is likely to damage the coils. High voltage in the coil with no path to take. Never run the engine with plug wires off or not gounded in some way. If you weren't planning to replace your coils you might really consider it now.
    Michael Stock, Trinity, NC
    R1100RT, R100, R60/6

  9. #9
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    that damn miss at 3500

    The floats height and/or float needles. You said you checked the bowl levels, but as lkchris said it's all main jet and my guess is that unless the plugs are black when you do a plug chop, you're going too lean and if the problem is worse when the motor is cold it tells you that you need more fuel. Don't ask me why I guess that.

  10. #10
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    ".................cutting out and stalling with accompanying backfiring...Checked points..........."

    Problem started suddenly makes me suspect the points - check again that
    0) The contact surfaces are clean and there is no whisker of metalic crap nearby that can screw things up
    1) The gap is not too large so maybe not enough contact time at higher rpm to charge coils
    2) There is no arcing between points wire clip attachment and the flyweights/springs at rpm (look running uncovered in dark)
    3) Maybe try different condenser

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