MONO

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 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 traveling 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 decel, 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 butterful 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 dimishes as the slide rises.)

An addition to this circuit is found on 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 decel 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 oulet 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) byhigh revs when the butterfyl is closed on decel.


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

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 . 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 caliper on a needle, you will find that it does not taper for the first 3-4 millimeters 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 restirction..

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 will in theory run up to around 3/4 throttle (mixture being controlled by the metering needle) with no main jet 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 aircleaner pod) where the air is relatively calm. Why? you don't want wind to be changing the airpressure 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 consistant 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.

POD FILTERS DO NOT SUPPLY LARGE STORED QUNATITIES OF STILL CLEAN AIR FOR FAST THROTLE RESPONCE ON CV CARBS ,PODS CAN ACT AS RESTRICTORS BY DELAYING THE CHANGE IN AIR PRESSURE