What is this part on my Bing carb; R 51

[Grant R26]

Can anybody tell me what this part is called and confirm its purpose please.

this appears to be a vent just forward of the slide on my Bing carbs. it has a tiny screen in it and vents to the atmosphere.

it is adjacent to the throttle stop idle adjust screw...called the "speed Screw" in the Bing book and the "throttle stop screw" in the BMW manuals

See photo please

Is it a vent to allow excess fuel out in case of flooding ?

Or an extra source of incoming air ?

I have the Bing manual...and excellent resource that I highly recommend...but no mention of it in the manual.

the carb numbers indicate that they are replacement carbs suitable to R51/2 and early R51/3 according to the Bing parts book

Grant in Toronto

[808Airhead]

That is the original air circuit for the idle circuit. You should have the old style mixture screws to go with that. Show us a pic of your mixture screws,I will tell you if they are correct. The later mixture screws have a "hat" on it that provides air in the circuit. I guess they were concerned with dust in there? Either way it is a visual cue that you have the correct early carbs on your bike.

[The Plunger]

Schutzschraube! Has a fine screen within to keep the dirt out. Yes, make sure you have original mixture screws.

[Grant R26]

thanks Thomas and Brian !!

attached are photos. I'm pretty sure you will tell me I've got the wrong mixture screws. But what does this mean ? I've driven the bike as a daily rider for 3 years now and it ran pretty darned good.

When I remove this "air circuit" with the tiny screen I see a passage way directly into the intake on the air cleaner side of the slide. I figured it was a vent of some kind to allow excess fuel out of the passage if it was flooding.

But...If it is part of the air supply for the idle circuit then I gather there must be a branch air passage that takes a T off this passage to supply air to the air jet ?

Is this correct ?

and if this is the case...does it matter that there is additional air supplied via the "hat" thay conceals the two air-supply intake holes in the screw ?

the actually amount of air introduced to the system would be metered by the screw to jet gap ? (approx 1.5 to 2 turns out)

So if I continue to use these incorrect air screws for the short term does it affect anything ?

thanks for the input.

Grant

[schrader7032]

I'm not sure what you're original picture showed...doesn't look right in my view. The second set of picture looks exactly like what's on my R69S.

The Bings before 1970 had an "air circuit" which fed the idle mixture supply. Beginning in 1970, the CV carbs were "gas circuits". So, turning the mixture screws CW or CCW caused change to lean or rich, depending on whether you were control air or gas introduction.

On the second picture, the small hat over the end of the needle screw was designed to prevent dirt/water to make a direct path to the carb. The hat never closed down on that small ridge, creating the gap for the air. The air had to follow a circuitous path to get to the hole that made its way into the carb circuit, thereby allowing for the dirt to drop out and not get into the carb.

[The Plunger]

Grant, that's the newer style adopted after the schutzschraube was deleted. See thread below

http://www.vintagebmw.org/v7/node/14884

Brian

[Grant R26]

Thanks Kurt...always grateful for your input.
these are the correct number carbs for R51/2 and early R51/3. My R51/3 was made in March of 1951
I've attached photos of the entire carb which should help you make sense of my questions

pictured on the underside of the carb beside the idle speed adjust is the mysterious vent with screen on the underside that is what I want to know about. Plus you can see the airjet screw with the "hat"

So...is this vent an air supply to the idle circuit with a branch that heads off to the Air Jet ?

and if that is the case, does it matter if I have the later version airjet screw with hat...as it is the pointed screw-tip contact with the jet that meters the air...does it matter if it is coming from two sources ?

if...as Thomas and Brian advise, it is an original /3 era air-mix intake, should I panic about changing out my existing airscrews ?

Interestingly, my "new" 1952 barn-find R51/3 also has those same little vents on the bottom side of the carb. it was last driven in 1962 so i'm figuring it is very original

thanks again for the help

Grant in Toronto

[Grant R26]

Whoa
thanks Brian. I'm seeing that now. I'll study it and see if my old brain can absorb it. Huge help

Grant

I'm still not sure if it matters if I have both the old vent and the second source of air at the later airscrew with hat?

[The Plunger]

Grant, I not sure if the redundancy will matter, as you said, you've been running with that configuration. Do the carbs have the brass needle seats or no?

[Grant R26]

Hey Brian
sorry I did not get back to you...side tracked with other bike projects.

the problem is solved and actually explained in full on the upper right of page 251 in the Barrington book. the carbs are the R51/2 which carried over a year or so into the early R51/3 bikes with that strange air source positioned on the intake.

I'm done worrying about it since the bike seems to run fine with it AND the later version idle jet screws ( ie ...two sources of air) and mystery solved

The redundancy has not been an issue and i won't loose sleep over it (grin)

thanks for the input

Grant in Toronto