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Old 03-14-2011, 09:19 AM   #1
mfp4073 OP
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How does the choke work on airhead bings?

Owners manual does not address this. I am used to manual chokes as found on classic cars/hot rods. Butterfly that restricts airflow thus making it richer that is adjustable from completely closed to completely open.

I know the bings dont use butterflies, its fuel passengers. Then realized that this may not be an adjustable setup. Are these only 3 positions (off, mid, all on choke) or are they graduatingly adjustable (is that a word)?

Does my question make sense? Not sure i am describing it very well.
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Old 03-14-2011, 09:26 AM   #2
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Its more like an enricher circuit. A separate circuit on the inboard side that essentially dumps lots o fuel past the idle inlet. Pretty sure. I would have to go peruse the Bing manual again to be sure of the particular pathway details, but its basically a fourth fuel metering circuit, used only at startup because it would flood the thing otherwise.
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Old 03-14-2011, 09:30 AM   #3
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Thanks for the reply, I got that much..but guess I didnt describe my question very well.

If off is 1, middle is 2, all on is 3, can i set it at say 2.5 in the controls travel? Or is it only a 3 position setup?
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Old 03-14-2011, 10:09 AM   #4
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Set it where ya want, there may be three holes but there are no notches. Once you've used it a time or two, your bike will tell you what works best for the initial fire. In the cold I crank mine on all the way, warmer whether is about 1/2.

Just remember to turn them off a few blocks down the road, as it is just a starting assist, not a "make a cold engine run better" like on a car.
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Old 03-14-2011, 10:14 AM   #5
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Weell, ok paraphrasing the bing book, the slide type carbs had a plunger assembly that let a predetermined combustible air/fuel ratio to enter the carb throat. THe CV carbs are similar but use a rotary valve instead, and the mix is varied by lever position.

Doesnt specifically say there are three positions, although you can feel detentes.
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Old 03-14-2011, 10:27 AM   #6
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Yeah can feel the detentes. Was not sure if there were passages that it lined up, so if you were not on the detentes it was blocking the flow of fuel to the enricher circuits. Since it does not say anywhere specifically "use settings at detentes" I would guess its trimable (again, is that a word?). BMW is pretty good at not leaving you guessing on stuff.
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Old 03-14-2011, 07:03 PM   #7
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I thought is was an air thingy allowing more air into the carburetor without using the slide/slide needle.
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Old 03-14-2011, 07:15 PM   #8
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I thought is was an air thingy allowing more air into the carburetor without using the slide/slide needle.
More air is the opposite of what you want. When its cold, you are richening up the mixture so it will run.
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Old 03-14-2011, 07:37 PM   #9
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More air is the opposite of what you want. When its cold, you are richening up the mixture so it will run.
So does fuel run through the choke body? I thought it allowed more air in to pull fuel it from the idle circuit.
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Old 03-14-2011, 07:42 PM   #10
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So does fuel run through the choke body? I thought it allowed more air in to pull fuel it from the idle circuit.
I dont know, I have not had the choke circuit apart yet!

The only thing I can figure is there is a circuit in there that opens a passage to allow more fuel in that is engaged when you turn the choke on. Please someone of knowledge correct me!
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Old 03-14-2011, 08:17 PM   #11
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So does fuel run through the choke body? I thought it allowed more air in to pull fuel it from the idle circuit.
It's not more air, it's some of the air. More air would require larger displacement, more rpm, or something along those lines. The side tracked air is used to pull up fuel from the enricher jet in addition to what is being pulled up through the idle circuit.

I forgot to answer that yes, the gas is pulled up into the enricher body and then with the air that pulled the fuel up goes into the carb throat via that big hole just in front of the throttle plate. It's the same air that was going to get there anyway but now it has a lot more gas in it.

supershaft screwed with this post 03-14-2011 at 08:31 PM
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Old 03-14-2011, 08:40 PM   #12
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Yeah can feel the detentes. Was not sure if there were passages that it lined up, so if you were not on the detentes it was blocking the flow of fuel to the enricher circuits. Since it does not say anywhere specifically "use settings at detentes" I would guess its trimable (again, is that a word?). BMW is pretty good at not leaving you guessing on stuff.

I think it is "trimmable" too. If not, in between the detents would take you back to zero, right? I feather mine as the bike warms up; unless of course I don't; at which point, I pull up to stop light and start cursing about "those damned airleaking carb tubes", because my idle is so high.
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Old 03-15-2011, 07:39 AM   #13
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Yep, redirected air

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Originally Posted by supershaft View Post
It's not more air, it's some of the air. More air would require larger displacement, more rpm, or something along those lines. The side tracked air is used to pull up fuel from the enricher jet in addition to what is being pulled up through the idle circuit.

I forgot to answer that yes, the gas is pulled up into the enricher body and then with the air that pulled the fuel up goes into the carb throat via that big hole just in front of the throttle plate. It's the same air that was going to get there anyway but now it has a lot more gas in it.
Glad SS took the time to type it out.
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Old 03-15-2011, 11:00 AM   #14
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It's like a mini carburetor piggybacked to the main one. That's why it's strictly called an enrichener circuit rather than choke.

It pulls fuel up from the float bowl through that brass tube in the corner. If the bowl gasket is compromised, an air leak will result along with loss of suction.

When the butterfly is closed, there's no venturi effect to draw fuel through the main carb jets, so there's a bypass circuit through the enrichener. That's the large opening in the carb throat entrance, and you can see it's pointed right at the enrichener. From there it exits on the engine side of the butterfly.

And yes, it will flow more fuel as the lever is gradually opened and none when closed (blocking off all air flow through that system).
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Old 03-15-2011, 02:52 PM   #15
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Ok, gotcha!

Sounds like we got that all worked out!

Calling it an enriching circuit makes more sense now that I got how it works. Thanks for taking the time to educate me (and anyone else that stumbles along to this)
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