Fuel is gravity feed to the carburetor from the fuel tank. At some point it flows through a filter to remove the micron size contaminates. The carburetor store enough fuel in a bowl to control the supply needed.
The supply is regulated by a needle valve and a float that goes up and down in the fuel. When the level reaches its maximum, the float is designed to close the needle valve, thus stopping any more fuel to enter the bowl.
The fuel is sucked from the bowl through jets and venture tubes. Works kind of like sucking on a straw. A flap in the lower section of the carburetor opens and closes the amount of vacuum the engine pulls into the engine.
This is called your throttle and is hooked to the cable on the throttle that is turned manually by hand to control the speed of the bike. The flap as it is opened allows the straw effect to pull the fuel needed through the jets into the engine. The jets regulate how much fuel to air ratio the engine can handle. The other circuit is the idle control.
It is regulated by a needle valve that is adjustable, and controls how much air or vacuum the engine pulls at idle. The carburetor has two sets of Jets. One for the throttle and the other for the idle circuit. The idle circuit is shorter thus allowing it to pull fuel all the time, while the throttle circuit is longer and takes a larger vacuum supply to activate.
The regulator between the two circuits works by a spring and an check ball. The vacuum for the throttle circuit will come into play only after the tension of the spring is overcome by pressure from the amount of vacuum allowed into the engine by the throttle plate.
Slide carbs offer a few advantages over a butterfly-style carb. First, and most importantly, the venturi size enlarges as the throttle is opened. This is a butterfly setup. Many early carburetors use this valve design. The shaft the disk is mounted on rotates 90 degrees or so. This position would be wide-open throttle. Slide carbs also have the advantage of not having throttle shaft bushings to wear out. Worn bushings can really make maintaining a reasonable idle speed and mixture a challenge.
Additionally, because that throttle shaft and butterfly are not taking up space in the carb mouth, a slide carb at wide-open throttle has no inherent obstruction in the intake path. Remember when we were talking about circuits earlier? One of the ways carburetors were improved was through addition of circuits. On the one hand, additional circuits provided increasingly granular and finer tuning ability.
The downside to that, as it is on anything with increased adjustability, is increased complexity, which brings the ability to adjust more incorrectly than ever before. Here is the orifice drilled into a pilot jet. It should be pretty easy to understand why gummy fuel or a dirty carburetor can prevent your motorcycle from starting and running.
One circuit that popped up and is found on most slide carbs is the jet needle we talked about earlier. On most slide carbs, the jet needle controls from about one-eighth throttle right up to wide open, with the pilot handling idle and off-idle duty, and the main circuit handling most of the bigger throttle openings, usually with some assistance from the jet needle. The jet needle. Note the various clip positions, as well as the very careful taper of the jet needle. Jet needles often have multiple positions for their retaining clips.
The higher that jet needle rides in the slide clip moving towards the pointed end of the needle jet , the richer the mixture can be made in the middle part of the throttle.
That handles the lower end of the midrange. The higher end is handled by the needle taper itself. A long, gentle taper will be leaner as the throttle is opened than a short, aggressive one as the needle moves up with the slide. You can still access the adjustment screw, you just have to remove the pressed-in plug, which usually qualifies as tampering with an emissions control device.
Kind of a Catch there, huh? That stumble generally occurs because airflow is increased suddenly, yet fuel lags behind. Accelerator pumps are basically a tiny, mechanically actuated fuel pump that is controlled by the throttle, and they usually open only under certain circumstances.
Another adjustment began to show up on carburetors, too, as time wore on: air bleeds. Adjustable air bleeds basically help hurry or delay the transition from one circuit onto another, again extending the adjustability of a carburetor, for better or for worse.
This is a CV carburetor. See that great big cover on top? That's your tipoff. Well, that sub-heading is a bit of a misnomer. Enter the constant velocity, or CV, carb. CV carbs have been around for a long time, but they became very popular in the s due to their ability to carburate cleanly while minimizing the excess unburned hydrocarbons that less precise fuel atomizing devices provided.
And this is a CV slide. Kind of sounds like a neat dance, doesn't it? This is a later diaphragm-style unit.
See why the tops on the carbs are so big? Effectively, the CV carburetor lifts the slide not mechanically, but pneumatically. The carb separates the slide lifting function by using the throttle cable to open and close a butterfly in the carb throat, rather than by lifting the slide directly.
The slide, now sealed with a diaphragm and kept closed with a weak spring, opens relative to engine vacuum. In this way, the carburetor slide is controlled by the engine. The rider is really controlling the airflow somewhat indirectly. Yes it would. Instead, a nice even rise in engine revs occurred in a fashion that was less damaging to the environment.
However, you generally will not see CV carbs usually identified by very large square or round tops where the diaphragms are contained on race or competition machines. Go check out a modern two-stroke dirt bike! Instead, their use was relegated primarily to more workaday standards and commuter motorcycles. CV carbs, as you may have guessed, are very thrifty on fuel. What they give up in throttle response and performance, they return in efficiency and economy.
Today on MC Garage we talk about the carburetor. For those of you who have an older motorcycle model or a modern two-stroke, one of the most confusing and intimidating tasks is probably jetting the carburetor correctly.
To some, it amounts to black magic. Step one is to understand how it works and what all the parts do. So how does the carb mix that fuel and air?
In simplest terms the air comes through the venturi and mixes with fuel supplied by the carb in a specific ratio. This is called the stoichiometric ratio. That ratio is theoretically In reality, your machine probably runs better at a richer ratio. Some tuners say This mixture is achieved using small orifices or jets to mix the fuel with the air. First and foremost, there is the place the fuel is pulled from: the float bowl. The float sets the level of the fuel from which the jets pull.
The float operates the needle valve, letting fuel flow in when the level drops and closes when the correct level is reached. On the bottom for the carburetor you typically have two jets.
The pilot, the smaller of the two, and the main jet.
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