Posted 09 November 2004 - 05:23 PM
First, the bottom end means the throttle closed. Not the speed of the bike or the RPMs of the engine. If you lug up a hill at low RPM but full throttle, it is not the bottom end for jetting descriptions. The throttle closed to 1/8 throttle position is controlled primarily by the pilot circuit. The pilot circuit is the pilot jet and fine tuned (after selecting the right pilot jet) by the fuel screw (aka pilot screw) in the bottom front outside of the carb.
For completeness, the midrange (throttle from 1/8 or so to just over 1/2 open) is controlled primarily by the needle taper and clip position. The full throttle or Wide Open Throttle (WOT) is controlled primarily by the main jet. For nitpickers, the throttle slide cutout is also a big factor in the throttle nearly closed position.
This is how I envision it, smarter people may clarify this, but with the throttle closed, there is high intake manifold vacuum in the front (cylinder side) of the carb. This is because the cylinder trys to suck in air and the carb slide is mostly closed. The pilot circuit is in the front of the carb, you'll see this because the fuel screw and pilot jet are toward the front and when you clean the carb, squirting carb clearner in either comes out in front of the slide. So when the manifold vacuum is high (higher the more closed the throttle), the manifold vacuum pulls fuel up the pilot circuit and feeds the engine. As the throttle opens, the manifold vacuum drops and the pilot circuit supplies less fuel. In fact, as the throttle opens, two factors make the pilot circuit less of a factor, the manifold vacuum drops and the fuel coming through the main jet is very large compared to the fuel the pilot circuit flows.
Most carbs have a butterfly valve that lets air through the carb as you open the throttle. The flat slide is more like a guillotine that opens as you open the throttle. There is a needle, maybe 4 inches long and tapered that slides down the flat slide and into the main jet. When the slide is closed, it either completely or almost completely blocks the main jet. As the throttle opens, the tapered needle pulls up out of the main jet allowing more and more fuel to be drawn into the carb. Both the needle and the slide open together. At some point, the needle is entirely out of the main jet and it doesn't get any larger with more throttle position. At this point to WOT, the main jet is the only control of the main circuit.
The main circuit is made up of the main jet (in sizes from 130ish to 180ish), the size and shape of the needle, and the clip position of the needle. The needle is held in position by a circlip type clip at the top of the needle. The needle has several grooves around it that the clip can be placed in. Selecting a high clip position puts the needle lower in the main jet and makes it leaner. Selecting a low clip position moves the need up higher in the main jet and makes it richer. Remember the needle is tapered so moving it higher puts a thinner, less blocking part of the needle in the main jet. Moving the clip is the normal way to jet the midrange but needles with different tapers or shapes are available.
Both the pilot jet and the main jet hang down into the fuel in the bowl of the carburetor. Vacuum draws the fuel up through the jets into the bore of the carb. For the pilot circuit, this vacuum is the manifold vacuum due to the engine sucking against a closed (or mostly closed) slide. For the main circuit (main jet and needle), the vacuum is venturi effect caused by the pressure drop of the air as it flows through the narrowest part of the intake tract. The more air that flows, the stronger the vacuum.
Since the fuel has to be sucked up from the level of the fuel in the bowl to the bore of the carb, the lower the fuel level in the bowl the less that flows for any given vacuum so low level leans the mixture. A high level richens the mixture. The fuel level in the bowl is controlled by bending the tab on the float.
There are two air jets, a pilot air jet and a main air jet. They supply emulsifying air for the pilot and main circuits. This sounds odd but a pure liquid stream of gas is hard to control because of surface tension, capillary action, and inertia. By mixing just a tiny amount of air, just enough that if you could see the fuel flowing in the jet, it would be maybe 1/8" of fuel, 1/8" of air, 1/8" of fuel etc. Like the flow through a straw as you suck up the last of a milkshake. The amount of air is neglible (1/2 by volume, maybe 1/100th by weight) so the air doesn't directly affect the mixture but it changes the weight of the stream delivered through the jet which affects the speed it comes out and the portion of air to gas that comes out. Again, this is just to make the fuel stream easier to control. Although it is useful for jetting, it was added to make the carb more predictable.
There are three other circuits. The choke circuit is another jet that drops down into the fuel and is only operational when the choke is out. Richens for starting and cold. The hot start circuit just dilutes the mixture with additional air (pirate air for you auto techs) to lean it when hot starting. And the accelerator pump to enrich on accelleration. The accelerator pump is literally a little diaphram single stroke pump bolted onto the base of the carb that pumps gas into the carb when the throttle is opening. One the throttle stops turning, the pump stops pumping. The pump is designed so it can pump enough for any application and is then limited for the exact application by letting part of the pumped gas leak back into the bowl instead of all of it going into the carb bore. This is controlled by the leak jet. A bigger leak jet will reduce the accellerator pump volume and a smaller one will increase it. A fuel pump is required on a non CV carbs because the vacuum falls rapidly as the open slide lets in a rush of air to fill the manifold and then stagnates briefly until the engine starts rev'ing to pull in more.
Hope that answered your question about the pilot and leak jet. Several years ago I wrote some long posts in the YZ250F/WR250F side called Jetting 101 and Accellerator Pump 101 that may still be there. I thought I was smarter then than I do now, so if someone can clarify or correct this, please do.
Mark Cantrell
p.s., just bought a 2005 CRF250 to replace an aging 2001 WR250F that is now for sale.
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