i really don't like poaching the internet for postings here. most of what you find out there is regurgitated crap with a different watermark on it, super weak. to that point, please realize that most of what you see here is original thought (mostly moronic thought really) and original pictures for better or worse. lately, however, i am very bored with work and all my toys are 2,400 miles away. this makes switching mental gears very difficult and so i am stuck on the same topic for way too long. anyways, it seems to help a little by posting up a few of the things i have been reading about and trying to fully understand.
THE VENTURI EFFECT
One phenomenon we to need to understand is that when a liquid or a gas is sitting still or moving slowly, the molecules that make it up are close together, making for a substance of normal density. For example, water flowing calmly down a wide stretch of a river is dense. But when this water hits a narrow canyon and rushes through it to maintain the flow, this rushing water "loosens up". Its molecules get temporarily farther apart, and the water thins out and becomes less dense relative to its normal density. And the faster it goes, the less dense it becomes.
The same is true for a gas. When air is still or moving slowly, it is dense and at "atmosphere pressure". But when it is forced to speed up faster than the surrounding air, it becomes thinner and less dense. We will call this condition a "depression". That is, air that is less dense than air at atmospheric pressure.The faster it goes, the greater the depression. And when a depression exists, air at atmospheric pressure wants to rush over to equalize the pressure.
This phenomenon, depression versus atmospheric air pressure is basic to the functioning of carburetors.
How do we get air to move through a carburetor? When the piston in our cylinder goes down, it drastically increases the volume inside the cylinder and creates a partial vacuum. Open the intake valve, and new air will rush in to fill this vacuum. Throw a carburetor in that path and the descending piston "sucks" air through the carb as well. And that's how we get a flow of air through the carb to work with. The vacuum created by the downward travel of the piston is actually a "depression", but we will call it a "vacuum" to differentiate it from the the depression we will be discussing which occurs in side the carb.
If the air path in the carburetor bore were the same size all the way through outside air could rush through the bore quite easily to fill the vacuum being created by the downward travel of the piston. It would therefore take high piston speeds (lots of pumping) to achieve enough air speed through the carb bore to thin the air enough to the create a depression. However, if an obstruction such as carburetor slide is placed in the carb bore the air path is now much smaller. The air stream has to speed up greatly to get through this bottle neck and outside air is largely blocked from helping to equalize the pressure. In this way, the air can be speeded up and a depression can be created at much lower piston speeds.
In carb speak, this bottle neck is called a "Venturi". As the incoming air speeds up to get past the Venturi, it thins out and loses density. We now achieve the "venturi effect" which is--depression at the point of the restriction.
How is the venturi effect used in the carb? We know that at the point of the depression outside air would love to rush in and equalize the pressure. For example we could drill a hole to the outside air at the point of the venturi and outside air would rush in. But the clever carb guys instead drill a hole which goes down into a bowl of fuel with outside air above it. The outside air can't get up through the hole (it is blocked by the fuel), but it can try, so it pushes on the fuel in the bowl, and forces some of that fuel up the pipe into the depressed air stream flying by the venturi. That plume of fuel coming out of the pipe mixes with the air rushing by, and that is how we get a fuel mixture to feed our cylinders to run our engines.
THE BUTTERFLY CARB
A simple carb might have just a butterfly type throttle valve and a carburetor bore that narrows in the middle to create a venturi. Open the butterfly and air flows, and fuel will be pushed up into the depressed air stream at the point of the venturi. This kind of carb is fine for things like lawn mowers, which run at a steady speed, under steady conditions-- where the need to accelerate is not a factor.
THE SLIDE CARB
The requirements placed on a motorcycle carburetor are much more complex. The engine needs to run well at a whole range of speeds. The engine needs to speed up and slow down. And the amount of fuel needed varies considerably. For example, the "ideal" fuel mixture is around 15 parts air to 1 part fuel. This theoretically provides just enough oxygen to fully combine with the fuel to produce a complete burn. But in the real world we need fuel ratios ranging from around 12:1 on the rich side to 18:1 on the lean side. This is because on the one hand, a richer mixture actually gives us more power to accelerate. And on the other, we can cruise steadily on a slightly lean mixture to give us fuel economy and low pollution.
The way motorcycyle carb designers tackled these problems was to replace the butterfly valve with a slide which is pulled up in the bore by means of the throttle cable. This slide did a couple of things. First, it provided a restriction in the bore to create a venturi. Since it can go up and down it is considered to be a "variable venturi". And second, the slide had a tapered needle on the bottom which it moved up and down in the "fuel hole" to vary the amount of fuel available from that orifice. At the slide's lowest position, the needle almost fills the hole, allowing little if any fuel to enter the small air stream. But as the slide is raised, the taper allows more fuel to pass into the air stream to combine with the greater amount of air now available. In this way a good air-fuel ratio can be maintained to meet the needs of the engine at different speeds and conditions (accelerating, cruising at steady speeds, etc.)
But there are a couple of problems with "slide carbs". First they can be a bit "touchy". That is, small changes in the slide throttle can give instant changes in speed, which means that holding steady speeds is sometimes tough. But the main problem has to do with quick acceleration. When you speed jockeys snap your throttles open suddenly, it presents a problem that the typical slide carb can't handle.
It goes roughly like this: you are going along at a steady speed, your engine making 3K revs. Your slide is open just far enough to keep air flowing by it fast enough to attract fuel (the venturi effect). Snap the throttle open and what happens? The slide moves up out of the way, and the air path is greatly enlarged. But the revs (and piston pumping action) haven't increased yet and the so the same amount of air as before is now being drawn through a much larger opening. What happens? The air stream slows down, the density goes up, and the venturi effect is momentarily partially lost. The outside air loses its motivation to push fuel up into this less-depressed air stream and a "lean" condition results (too much air, too little fuel). The engine coughs and stumbles until the revs can pick up enough to achieve sufficient air speed through the bigger opening to restore the venturi effect. At which point you finally take off like the scalded animal of your choice.
One way to deal with this is by adding an "accelerator pump" which provides a shot of fuel as the throttle is opened. But there is another way which has been adopted for use in almost all modern carbureted street bikes:
CV (Costant Velocity) CARBS
Both Hitachis and Mikunis are CV carbs.
In respect to these carbs, I'll be talking about the intake side, and the exit side. Air comes into the intake side and exits out of the engine side as fuel mixture.
The CV Carb has a more complex air control system than the two carbs described above:
---The butterfly valve is back, and sits toward the engine side of the carb. It is opened and closed by means of the throttle and throttle cable and controls the amount of air that can flow through the carb.
---But the slide is retained. It sits in the middle of the carb on the intake side, before the butterfly. But instead of being pulled up and down by the throttle cable as in the slide carb, it now has no direct connection to the throttle cable at all. It is now attached to a rubber diaphragm and is raised and lowered by vacuum (depression) introduced on the top side of this diaphragm through holes drilled up through the slide. The slides in Hitachis are round, and in Mikunis they are flat.
Now we'll try to figure out how CV carbs work.
When the butterfly valve is closed, very little air is moving in the carb bore. (The engine is getting some air and fuel through the pilot circuit, which we'll describe later.) With little to no air flowing, the air in the carb bore and the air in the closed chamber above the diaphragm are at close to the atmospheric pressure of the outside air.
Open the butterfly, and several things happen.
1. Air now speeds through and venturi effect (depression) at the point of the slide (variable venturi) is created.
2. The depression at the venturi is transmitted up through the holes in the slide to the closed chamber above the diaphragm. This lowers the density of the air in that chamber.
3. The open air below the diaphragm now wants to rush into that chamber to equalize the pressure, but it can't because there is no passage.
4. So it does the next best thing and tries to push its way in through the underside of the diaphragm.
5. The diaphragm can't let the air in, but it is flexible so gives way it is pushed up by the outside air pressure.
6. As it goes up, it pulls the slide with it, and the slide pulls the tapered fuel needle up in the fuel hole.
7. More air flows, more fuel is pushed into the air stream, and the engine accelerates or runs at higher revs.
But how does this improve things over the simpler slide carb?
When the throttle is cranked on the slide carb, the slide is pulled up immediately by the throttle cable, expanding the variable venturi suddenly, and causing the lean stumble described above.
When the CV butterfly is opened, the slide does not immediately jump up to a much more open position. It raises gradually as the increasing engine revs provide the needed depression (at the venturi), which is then transmitted to the chamber above the diaphragm. As the slide rises, the increasing depression also encourages more fuel to enter the carb bore and combine with the greater air supply now available. And the higher the slide goes, the more fuel the tapered needle permits to flow. In other words the genius of the CV carb is that the fuel from fuel hole can now "keep up" with the increasing air available--maintaining the mixture at proper ratios during the accelereation process.
In summary, the CV carb provides quick enough acceleration (no lean stumbles to slow things down) which is also smooth. And overall we get a "kinder, gentler" carb which gives us less twitchy responses as we make small throttle adjustments.