Virago Carburetor CV

Mikuni Carb(View)                                                                        Early Hitachi Carb(View)

 

The Virago Carburetor some wise individual said this:”Make things as simple as possible, but no simpler.” I try to keep things simple, but some aspects of motorcycle carburetion are a little complex and to really understand what is going on we have to struggle to master them. So I hope motivated readers will hang with me as we go through this and maybe even enjoy some of it.

The first Viragos to hit our shores in 1981 had Hitachi carbs on them, and Hitachis lasted on the 700, 750, 920, 1000, and 1100cc models through 1987. In 1988 Hitachis were replaced by Mikunis. In discussing these carbs in the other carb articles I am going to assume a certain amount of basic understanding on the part of the reader. If you are starting from scratch, this article should be helpful in describing what goes on inside carbs. And especially “CV” (Constant Velocity) carbs, which is what all these carbs are. For learning specifically about the carbs in your bike, my Hitachi and Mikuni carb articles, plus a service manual, should be helpful. Also, for those who want to go further, Haynes offers a manual on motorcycle carburetion which will deepen your understanding of carbs. Much of the information here was learned from reading that manual.

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 Virago 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 motorcycle 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 slides 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 (Constant 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 acceleration 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.

Now we’ll get into:

FUEL CIRCUITS

Carbs are such that they cannot meet all the different running situations with one single system. Starting, idling, acceleration, deceleration, and steady running all impose different fuel requirements on carbs. Carbs also have to handle different engine speeds, different loads, different engine temperatures and other variables. So we find that CV carbs need three distinct “circuits” or fuel delivery systems to meet these different needs. The three circuits are:

The starting circuit, the pilot circuit, and (what I call) the run circuit.

STARTING CIRCUIT

The starting circuit (often called the choke circuit–but it doesn’t really “choke” anything) provides a special fuel supply needed to start the engine, when the engine is cold.

Why does the engine need a lot of fuel to fire when it is cold? For a fuel mixture to ignite, it needs to be made up of very tiny (atomized) particles of fuel suspended in the air. Cold fuel tends to stay in big drops which don’t ignite easily. Also these big drops tend to cling to intake walls. So in a cold engine, a lot of the fuel doesn’t atomize correctly and is just wasted. Therefore you need more fuel to start with to make up for these losses, and assure that enough of it is atomized to give you a mixture which will ignite properly. As the engine warms up, atomization becomes much better and more complete, so less fuel is needed to create the proper air/fuel ratio, and the start lever can be let off.

The starting circuit is really a separate system in the carb. It takes its air from a port in the bore which is located in the main air path before the slide and the butterfly. It gets its fuel from a separate tube running into the float bowl. When the start lever is pulled at the handlebar, a plunger is lifted which opens the air and fuel passages. The engine is then cranked, and since these passages are small, you get you get the needed air speed going through them with enough venturi effect to draw fuel from the bowl into the air stream. This mixture exits from a port on the engine side of (after) the butterfly and goes on into the cylinder.

This circuit, if working properly, is designed to provide the proper mixture all by itself to start the bike. Hence the admonition; “start your bike with the throttle closed.” (Note also that closed throttle starting is also easier on the starter, since the piston is not pulling in a full gulp of air. Compression is about half of normal and the starter doesn’t have to work as hard to crank the engine.) As the bike warms up, the plunger has a half open position which cuts the fuel back but leaves the air open, leaning out the mixture. On full warm up the plunger is closed and cuts off both passages. Note that different bikes have different starting habits, and a quick blip after the engine catches helps with some of them. Outside temperature can also affect how you use the choke. (O.K. I’m calling it the choke) More choke on cold days, less on hot days.

PILOT CIRCUIT

This is sometimes called the “idle circuit” but it does a lot more than control idle. And it is perhaps the most misunderstood of the three circuits. The role of the pilot circuit is basically to run the engine when the throttle is closed, as when the engine is idling or the throttle is closed on deceleration. But this circuit is also the main source of fuel at very small throttle openings. As the throttle is opened past 1/4, the importance of this circuit diminishes, as the main fuel supply is now provided through the main “fuel hole” and controlled by the needle/needle jet and ultimately the main jet. But the pilot circuit does remain active and makes a (progressively smaller) contribution all the way to WOT.

The typical set up is this: The pilot circuit get its fuel from the float bowl through the pilot jet. The circuit also has a pilot air jet, but the purpose here is not to provide all the needed air (as in the starting circuit), but to provide air to premix with and partially aerate the fuel before the mixture enters bore and completes the atomization process with air travelling through the bore. Such little air passages are sometimes called “air bleed” circuits.

This circuit typically has two outlets. One is called the “pilot outlet” and is located on the engine side of the butterfly valve. This outlet supplies the fuel mixture to support idling and deceleration (that is, off-throttle running). It has an adjustment screw which controls the amount of fuel mixture entering the bore under off-throttle conditions. This screw is sometimes mistaken for an “air screw” but it is not. Turning in (right) reduces the amount of fuel mixture, and turning out (left) increases it. In for lean, out for rich.

The other outlet is call the “bypass outlet” and is located right at the point in the bore where the bottom of the butterfly comes to rest when closed. Typically, the butterfly is set a tad open to permit just a slight amount of air to pass by at the bottom to support idle and deceleration, and most of the fuel for these functions is supplied through the pilot outlet. But as the butterfly is opened, more air flows past it, and the venturi effect starts to work on the bypass Additional fuel is now drawn out of the bypass to support low speed running and cruising at small throttle openings. (Note that if the butterfly valve is adjusted to ‘fully closed’ the engine will probably not start or idle. It needs to be open a tad. As mentioned above, these outlets continue “giving” throughout the rev range, but their contribution to the overall mixture diminishes as the slide rises.)

An addition to this circuit is found on later Hitachi and all Mikuni carbs. This is the “coasting enricher”. A typical problem in earlier carbs was the fact that when you chopped the throttle (closed the butterfly) on deceleration, there would not enough fuel in the mixture at the (at that moment) high revs to allow the engine to fire consistently. You would then get a “lean misfire”. That is, the engine would fail to fire, and the unburned mixture (lean though it was) would enter the exhaust header. Then when the engine next fired, you’d get a backfire. (So backfiring on deceleration is typically a lean condition, and not “loading up” as some people think.) The solution they came up with was to reduce the amount of air in the “airbleed” circuit by about half, meaning the fuel content hitting the bore from the pilot outlet was much higher than the normal idle fuel mixture you get on closed throttle. Once the revs came down, the full air bleed would be restored for proper idling. The “coaster enricher” is activated by the strong vacuum created in the carburetor holder (intake stub) by high revs when the butterfly is closed on deceleration.

On Hitachis some external piping was added to service these diaphragm driven valves. On Mikunis the needed passages were drilled into the carb bodies.

Of special note: we now understand that those adjustment screws only affect the pilot circuit, and mainly the mixture on idle and deceleration.

FUEL LEVEL IN THE FLOAT BOWL

This is not a circuit but will influence running, particularly at lower speeds. As a general comment, lower will make things leaner, and higher will make them richer. This factor works in conjunction with pilot jet size and pilot screw settings. Lacking professional tuning equipment, we generally accept/use the factory settings

THE RUN CIRCUIT

This circuit takes its “airbleed air” from the main air jet, and its fuel from the float bowl. The fuel travels up through the main jet, and is pre-mixed with air from the main air jet in the needle jet (called by Yamaha in the Hitachi years the “main nozzle”). This needle jet is a long jet with holes in the side to permit the air to enter and be mixed with the fuel–before this mixture plumes out into the main bore to be further atomized as it heads to the cylinder. The tapered metering needle rides up and down in the needle jet and meters out more fuel the higher it goes. At wide open throttle (WOT) the slide and needle are fully raised and the needle is effectively “out of the way” in the needle jet, allowing maximum fuel to flow into the carb bore, regulated only by the size of the main jet.

When does the needle taper cut in? If you put a digital calliper on a Hitachi needle, you will find that it does not taper for the first 3-4 millimetres residing down in the needle jet. So presumably until the slide raises more than 3-4mm, we are still in “pilot circuit county”, since with no taper the needle jet is pretty well filled with the needle and little additional fuel will be pushed out that hole. Once the taper cuts in additional fuel starts to flow out and this progressive metering continues until the much steeper taper of the needle drops it out of the game as WOT is approached, and the main jet becomes the only restriction..

Note that jet changes typically involve the main jet, and to a lesser extent the pilot jet. In these carbs we never seem to get into the air or needle jets.

An interesting thing to do is to make some marks on your throttle showing its position at 1/4, 1/2, 3/4 of rotation and WOT. You’ll be surprised to see how little the butterfly is actually open at steady cruising speeds. You’ll see some serious throttle rotation on brisk acceleration, but just try to maintain a steady speed at high throttle openings on the freeway. This will get you too much speed or too many speeding tickets, whichever comes first.

So those are the three circuits. Half the trick is diagnosing carbs is to figure out fuel, air and outlet paths for the various circuits. Note there is a degree of independence between the pilot circuit, the needle/jet, and the main jet. For example, we are told that the engine will start and idle on the pilot circuit with no needle or main jet in the carb at all (I’ve never tried it). Also, the engine could, I suppose, run up to some point (mixture being controlled by the metering needle) with no main jet in place. (Not to be tried with Hitachis, since the main jet screws into the needle jet and holds it in place.)

CARB VENTING

As we have now seen, the CV carb needs the presence of outside air (at atmospheric pressure) inside the carb for several reasons:

–Outside air is needed under the diaphragm to push it up.
–Outside air is needed above the fuel in the float bowl to push down on the fuel and force it up past the various jets and into the starting, pilot and run circuits.
–And outside air is needed to service one or more of the air jets that reside inside the carb body..

How does this outside air get in? It gets in through the breather pipe which sits toward the top of the carb just under the diaphragm. Air jet(s) also reside in this space and have access to it. And drillings in the carb body allow the outside air to go down and enter the space above the fuel in the float bowl.

A breather tube typically attaches to the breather pipe and extends to some point (e.g., behind a side cover, or inside the air cleaner pod) where the air is relatively calm. Why? We don’t want wind to be changing the air pressure at the end of this pipe, because this will disturb the operation of the air jets, the diaphragm, and the fuel delivery. These functions need consistent outside air pressure to work properly against the various levels of depression created in the bore, above the diaphragm, etc., as the carb goes about its work.

TUNING CHALLENGES

A stock Virago should run pretty well with stock carbs. But when you start playing with aftermarket pipes and air cleaners, some retuning will be called for. For the average rider, tuning Virago carbs up to professional standards is not that easy. First of all, the carbs are somewhat of a hassle to pull off and remount (although you get faster as you do it). In theory, the instructions you get with tuning kits have been worked out and tested. So you maybe able to get good results quickly by simply installing the kits per instructions. But if you are “on your own” so to speak, and your only only test pad is your bike, then you have to pull the carbs to make each change (jets, etc.), then remount the carbs and take a ride to see what you’ve done. Going to a professional tuner who has a Dyno machine is expensive, but maybe worth it, if you want the best tuning result. For those folks that want to see what professional tuning is really all about, go to Google, find Factory Pro, and check out their outstanding tuning page.

CONCLUSION

Hopefully we now have a general idea about what is going on inside our Virago carburetor. The circuits in Hitachis and Mikunis are pretty much the same. For more specific information on tuning, problems, and fixes, see the article covering your carbs.

Posted 4/05

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