Carb vacuum balancing question.

Discussion in 'Airheads' started by bushyb, Dec 7, 2012.

  1. bushyb

    bushyb Adventurer

    Joined:
    Apr 14, 2012
    Oddometer:
    74
    Location:
    Rotorua, New Zealand
    Carb vacuum balancing question.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:eek:ffice:eek:ffice" /><o:p></o:p>
    <o:p> </o:p>
    What causes one Bing Constant-depression carburettor to have a far greater vacuum than the other when connecting a manometer on 1982 R100RS? Where should I be looking?<o:p></o:p>
    <o:p> </o:p>
    I have re-routed the cables and they work mush better, done and checked the timing and checked and set the valves which werenÂ’t out much at all. Checked all the head and rocker nuts, all seems okay. But I am sure when I start her up again, the vacuum is still going to be stronger on the one side than the other side. What else can I check I believe the carbs are good. Oh I have also used a single vacuum gauge on each carb independently before trying the manometer before I checked the valves and the one side was very erratic while the other side had smooth constant pulses. Any other ideas? Thanks<o:p></o:p>
    <o:p> </o:p>
    #1
  2. Renner

    Renner combustophile

    Joined:
    Jul 11, 2007
    Oddometer:
    2,762
    Location:
    sunny SoCal
    'need to ask: does it respond to adjustments at the idle speed adjustment screw?
    #2
  3. David R

    David R I been called a Nut Job..

    Joined:
    Oct 16, 2009
    Oddometer:
    2,802
    Location:
    In Da Swamps of WNY
    Erratic vacuum = leak

    Sent from the phone in my shoe. Maxwell Smart.
    #3
  4. Wirespokes

    Wirespokes Beemerholics Anonymous

    Joined:
    Jul 26, 2002
    Oddometer:
    8,697
    Location:
    Jackson's Bottom Oregon
    The side with the butterfly open the farthest will have the least vacuum.

    First step is to loosen the cables - give them some slack. It's possible to lower the idle speed adjustment, but the cable won't allow the butterfly to close any more.
    #4
  5. bushyb

    bushyb Adventurer

    Joined:
    Apr 14, 2012
    Oddometer:
    74
    Location:
    Rotorua, New Zealand
    I warmed up the bike a bit at first knowing of the problem in order to try finding what was the cause. The initial settings of the mixture was done by ear on both carbs so the adjustment did respond. I then cut the engine and disconnected both the throttle cable and choke cable and started the bike. And found that it was idling a bit high which it was in the begging. I then backed out the idle screws all the way and it dropped down to about 850 rpm.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:eek:ffice:eek:ffice" /><o:p></o:p>
    <o:p></o:p>
    Cut the bike and connected the vacuum tubes and started the bike and it immediately sucked the one side straight up over the high point in the meter, and immediately cut the switch. Now I am scratching my peanut??? <o:p></o:p>
    <o:p></o:p>
    My thinking was that if engine is warm and there is no pressure on the carbs from any of the cables or adjustments screws it would be easy to set and find the cause.

    <o:p>Now thinking back on it perhaps I should have tried adjusting the mixture with the single dial gauge before relying on the EAR mixture setting before attaching the monometer. </o:p>
    #5
  6. Renner

    Renner combustophile

    Joined:
    Jul 11, 2007
    Oddometer:
    2,762
    Location:
    sunny SoCal
    there's no "high pressure" (beyond atmospheric)... the fluid shot toward the low pressure.

    turn clockwise the idle speed screw on the side the fluid went toward.

    this will open the throttle plate, allowing the manifold pressure on that side to rise.
    #6
  7. bushyb

    bushyb Adventurer

    Joined:
    Apr 14, 2012
    Oddometer:
    74
    Location:
    Rotorua, New Zealand
    Thanks, I have had another look. It seems the problem could be the left carb, with everything disconnected and engine warm adjusting the mixture on that carb makes no effect to the idling. I can close the mixture screw all the way and there is no change. So I am assuming the must be a leak on that left carb. Would that be correct?<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:eek:ffice:eek:ffice" /><o:p></o:p>
    <o:p> </o:p>
    I know that the diaphragms are in good condition. Might have to replace all the O rings?? Any other Ideas?<o:p></o:p>
    #7
  8. bmwrench

    bmwrench Long timer

    Joined:
    Nov 10, 2006
    Oddometer:
    3,455
    Check the valve clearances.
    #8
  9. bushyb

    bushyb Adventurer

    Joined:
    Apr 14, 2012
    Oddometer:
    74
    Location:
    Rotorua, New Zealand
    <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:eek:ffice:eek:ffice" /><o:p> </o:p>
    Thanks I have done the valves and timing.<o:p></o:p>
    #9
  10. Wirespokes

    Wirespokes Beemerholics Anonymous

    Joined:
    Jul 26, 2002
    Oddometer:
    8,697
    Location:
    Jackson's Bottom Oregon
    Check the idle jet and mixture o-rings. They often get split by the sharp edges of the o-ring groove. I chamfer the edges to prevent that.
    #10
  11. supershaft

    supershaft because I can

    Joined:
    Sep 9, 2010
    Oddometer:
    9,116
    Location:
    San Francisco Bay area
    If your carb's idle circuit isn't leaking you have a valve that is leaking. A leak down can diagnose that for sure. I have found many a leaking valve via a inop mixture screw.
    #11
  12. georgesgiralt

    georgesgiralt Been here awhile

    Joined:
    Aug 12, 2012
    Oddometer:
    141
    Hello !
    Do you have rebuild your carbs ?
    If not, I suggest you do that at once. It will save time and money. Change all O-rings including the one on the butterfly pivot, clean all passages in the carbs body and change all jets and jet needles (buy the exact same size as is originally shipped with the bike) Then if the rubber tubes at the cylinder head holding the carbs are old, change them.
    The carbs will be easy to adjust and give you a high mileage per gallon paying back the money spend in their rebuild...
    #12
  13. boxerboy81

    boxerboy81 Stay Horizontal

    Joined:
    Jul 22, 2009
    Oddometer:
    3,077
    Location:
    Melbourne, Oz
    Has the left mixture screw also got the small spring?







    .
    #13
  14. cycleman2

    cycleman2 Been here awhile

    Joined:
    Jan 18, 2010
    Oddometer:
    608
    Location:
    Alberta, Canada
    If nothing is changing when you are adjusting the various screws than it is not getting enough air/fuel.

    As others have said, loosen the cables until you have slack at each carb. Set each throttle plate screw to 1 turn from contact and then set the screw that is in front of the float bowl closest to the intake, to the factory setting which would be around 3/4 to 1. I could look it up but that will be close enough.

    Hook your gauge to each vacuum port on the carb and start the bike using the choke. Let the bike run for a bit until it will run off choke. Then carefully turn each carbs screws in and out, as you would when sync the carbs. Leave the cables alone for now as you want to get the engine to idle cleanly with maximum equal vacuum. Once you have the carbs balanced at idle then you move on to adjusting the cables so that you get the same pull at each throttle plate.

    Sometimes people get the atomizer in backwards after a carb rebuild. The bike will run but not well as its not getting enough fuel. Also check that both cylinders exhaust is generating heat.
    #14
  15. bushyb

    bushyb Adventurer

    Joined:
    Apr 14, 2012
    Oddometer:
    74
    Location:
    Rotorua, New Zealand

    Yes
    #15
  16. bushyb

    bushyb Adventurer

    Joined:
    Apr 14, 2012
    Oddometer:
    74
    Location:
    Rotorua, New Zealand
    Thanks Done
    #16
  17. bushyb

    bushyb Adventurer

    Joined:
    Apr 14, 2012
    Oddometer:
    74
    Location:
    Rotorua, New Zealand
    Thanks, not quite sure of the comment above but you might be onto something. I removed the left carb and on removing the main jet and nut the needle jet just fell out but the atomizer remained in place. Now I am not sure that this would cause the problem that I have as the main jet when screwed in would hold the needle jet in place. Any ideas on how to remedy this? Thanks <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:eek:ffice:eek:ffice" /><o:p></o:p>
    #17
  18. boxerboy81

    boxerboy81 Stay Horizontal

    Joined:
    Jul 22, 2009
    Oddometer:
    3,077
    Location:
    Melbourne, Oz
    If the needle fell out then it can't be held in correctly. Maybe the holding clip needs replacing?
    #18
  19. disston

    disston ShadeTreeExpert

    Joined:
    Mar 22, 2011
    Oddometer:
    12,314
    Location:
    Silver Spring, Md
    He said the "needle jet" fell out. The "jet needle" is held in place by a clip. Notice in the former it is a "jet" the modifier being needle so it's not confused with the main jet. And in the later it is a needle the modifier being jet so it is not confused with the "float needle" or something else.

    The atomizer and the needle jet are held in place by the venturi. Sometimes the atomizer will fall out and sometimes it is stuck because it is bent. They are very thin.

    Notice the venturi in some parts micro-fiche pictures does not seem to have an O-ring;

    [​IMG]

    In the above picture there is no O-ring shown on the venturi. But there is an O-ring shown on the idle mixture screw and in the blub for these parts (not shown at all in my reproduction) there are two part numbers for #13. One is the idle mix O-ring. The other is the venturi O-ring.

    I had weird symptoms until I figured this out because my favorite parts guy told me there was no O-ring on the venturi. There is an O-ring on the venturi and now he knows also.
    #19
  20. motog

    motog Been here awhile

    Joined:
    Feb 7, 2012
    Oddometer:
    129
    Location:
    Upper Cumbucker West
    Most probably. Tear the carb down, rebuild it with all new parts. Have had the same problem on several carbs over the years. New o rings, jets, gaskets etc has always fixed it. The parts aren't expensive but it is a half day job and I've always done both carbs at the same time.

    Jim
    #20