MONO

CV (Costant Velocity) 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.


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 I'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.

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. More choke on cold days, less on hot days.