Bing Vacuum physics

[tomjohnston]

Since the piston draws air through the carb, and the resulting vacuum causes the diaphram to lift the slide and needle open.... if a low resistance air cleaner is used, does the lack of a resistance to help create that vacuum keep the slide from rising as far as it should? My '82 R100 pulls like crazy but mellows out at 4000 rpm and won't go much higher. Before the new air filter, it was much happier .

[Ken in Oklahoma]

Since the piston draws air through the carb, and the resulting vacuum causes the diaphram to lift the slide and needle open.... if a low resistance air cleaner is used, does the lack of a resistance to help create that vacuum keep the slide from rising as far as it should? My '82 R100 pulls like crazy but mellows out at 4000 rpm and won't go much higher. Before the new air filter, it was much happier .

I think it would be much more accurate to say that rotating the throttle causes the butterfly to open, which routes a vacuum signal to the diaphragm, which lifts the slide, which allows the air to go through the carb, which enrichens the air with gasoline.

With that scenario in mind I can't imagine a way that a less restrictive air cleaner could reduce the vacuum on the diaphragm.

Tell us about your low resistance air cleaner? As compared to the stock filter? Are you trying to accomplish something, such as a performance boost with a less restrictive air cleaner? Did you do any other work on the bike at the time you put in the less restrictive air cleaner? Did you make any changes to the jetting? You get the idea.

Perhaps with a bit more background I or others might be able to help you work the problem. We delight in problems we think we know something about.



Ken

[Kurt in S.A.]

I would think that the air filter doesn't ultimately control the volume of air into the carb throat. As Ken says, it's the butterfly and also the pistons are the thing that is creating the need for air to flow through the carb as they move to BDC on each stroke. You're not going to get any more air or speed of the air than what the engine needs at any given moment. Obviously, if the shape of the venturi changes, that can create some differences in vacuum created.

Kurt in S.A.

[tomjohnston]

I guess I pictured a simpler diagram in my mind. Like drawing air through a straw. Air flows, but nothing else. Put the straw in water, the resistance of the water creates vacuum and the liquid goes up. Get a super-thick milkshake, you have to create more vacuum to get it to move. You don't create vacuum without applying resitance. I figured a certain amount of vacuum being created by a certain air cleaner(milkshake)...less resistance (water)...less vacuum... less travel of the slide and needle.

[Kurt in S.A.]

One the incoming air is past the filter, it's still just air to the carb. The only thing a filter is going to do is reduce the amount of air that can get through...think dirty filter. In general, the stock filter in good condition will not limit air to the carb...if you have something that has less resistance (ie, I guess you mean it doesn't filter as well...what's up with that? ), that also will not limit the air to the carb.

Kurt in S.A.

[tomjohnston]

Don't panic guys.........I'm putting the air filter back.......I was just looking at things and started to wonder why....................and so on

[Ken in Oklahoma]

Don't panic guys.........I'm putting the air filter back.......I was just looking at things and started to wonder why....................and so on

Damn! Couldn't you have held back on that just a bit longer Tom? There I was, rehearsing an essay in my mind how a vacuum doesn't suck air through the carbs. Rather atmospheric pressure blows the air through the carbs.

Life is so unfair sometimes. Sigh!



Ken

[Major Softie]

Tom, your original post shows a misunderstanding of where the vacuum comes from that explains why you thought the lower restriction air filter could change the vacuum, while others posting here saw that it would not.

The vacuum that lifts the slide does not come from reduced pressure in the carb due to the engine sucking in air past a restriction (the air filter). The vacuum that lifts that piston is created by the venturi effect of air being drawn through the carburetor venturi, accelerating through that venturi, and thus reducing the air pressure inside that venturi. It's the same way a wing works: making air move faster across the top surface of a wing, reducing the air pressure on the surface that is accelerating the speed of that air.

Anyone with a 3rd grade education can see that putting a restriction in a moving column of air will increase the pressure against that restriction, but it turns out that our 3rd grade educations left out some interesting properties of physics.

[chasbmw]

If you are topping out at 4000 rpm I would think that the float levels are too low

[tomjohnston]

WOW! Major Softie needs to work on his bed side manner. I don't need to be hit with the "third grade education" stuff.I know all about Bernoulli's laws......Since Bings are so sensitive I thought the addition or removal of restrictions might throw that balance off enough to bother things. Sorry !!!!