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bushyb 12-07-2012 01:51 AM

Carb vacuum balancing question.
Carb vacuum balancing question.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><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>
I have re-routed the cables and they work mush better, done and checked the timing and checked and set the valves which weren’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>

Renner 12-07-2012 05:27 AM

'need to ask: does it respond to adjustments at the idle speed adjustment screw?

David R 12-07-2012 05:29 AM

Erratic vacuum = leak

Sent from the phone in my shoe. Maxwell Smart.

Wirespokes 12-07-2012 05:40 AM

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.

bushyb 12-07-2012 10:56 AM

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:office:office" /><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>
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>

Renner 12-07-2012 11:02 AM

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.

bushyb 12-07-2012 06:13 PM

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:office:office" /><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>

bmwrench 12-07-2012 06:21 PM

Check the valve clearances.

bushyb 12-07-2012 08:25 PM

<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>
Thanks I have done the valves and timing.<o:p></o:p>

Wirespokes 12-07-2012 10:38 PM

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.

supershaft 12-07-2012 11:17 PM

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.

georgesgiralt 12-08-2012 01:15 AM

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...

boxerboy81 12-08-2012 01:31 AM

Has the left mixture screw also got the small spring?


cycleman2 12-08-2012 08:30 AM

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.

bushyb 12-08-2012 10:28 AM


Originally Posted by boxerboy81 (Post 20203265)
Has the left mixture screw also got the small spring?.


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