The carburetor is a device that mixes air and fuel for an engine.
The carburetor works on Bernoulli's Principle: the fact that moving air has lower pressure than still air, and that the faster the movement of the air, the lower the pressure. The throttle does not control the flow of liquid fuel. Instead, it controls the amount of air that flows through the carburetor. Faster flows of air and more air entering the carburetor draws more fuel into the carburetor due to the partial vacuum that is created.
A carburetor basically consists of an open pipe, a "throat" or "barrel" through which the air passes into the inlet manifold of the engine. The pipe is in the form of a venturi — it narrows in section and then widens again, causing the airflow to increase in speed in the narrowest part. Inside the venturi is a throttle slide — this valve controls the flow of air through the carburetor throat, thereby regulating engine power and speed.
Carburetor Theory: The carburetor sucks fuel out of the bowl by the “venturi effect” that Bernoulli figured out. This means forcing air through a narrowed "throat" (the venturi or narrowed tube through the carb) makes the air speed up. Fast moving air is lower in pressure than the “outside air” pressure. Since we maintain the fuel bowl at “outside air” pressure by venting it on top, the fuel in the float bowl is under the higher "outside air" pressure than the low pressure air rushing through the carb throat. A pipe runs from the bottom of the bowl, through the main jet, to the area of the fast rushing air where the pressure is lower. The "outside air" pressure in the float bowl pushes the fuel to the lower pressure area. That's the middle of the throat of the carburetor. The fast moving air rips off tiny droplets of fuel from the pipe which mix with the air and flow to the engine to burn.
Moped carburetors work under the same principles as larger carburetors, but are usually simple and have fewer moving parts. Several elements of carburetion, such as the idle circuit, are built into the carb body and non-adjustable. This saves on cost and requires less maintenance, and the smaller carburetors are less susceptible to atmospheric factors and so do not suffer for this lack of adjustment.
Carburetor Parts & Functions
See also: Carb Operation/Cleaning
- Main jet
- Part of the main fuel supply circuit, the main jet is basically a screw with a very accurately sized hole through it. This hole's size determines the maximum flow rate of fuel into the venturi. At WOT the carburetor uses the main jet's full flow capacity. It is the main passage of fuel from the float bowl to the carb throat.
- Startup jet
- Part of the Startup circuit, the startup jet supplies a proper amount of fuel in the short period of time when the throttle is slightly open and the idle circuit still supplies a significant amount of fuel.
- Idle jet
- Part of the carburetor's Idle circuit, the Idle jet controls the maximum amount of fuel that the idle circuit will supply. Sometimes, after replacing a carburetor with a more elaborate one, the idle jet might be too rich to let the engine idle correctly.
- Idle mixture screw
- Idle screw
- Adjusts a slight amount of throttle slide displacement at closed throttle. Controls idle engine speed.
- Through the midrange throttle application(not closed and not wide open), the tip of the needle regulates the flow of fuel out of the main jet by blocking its opening. Typically adjusted by changing the position of a snap ring, it controls the air/fuel mixture while midranged throttle is applied.
- A spring loaded gate valve which is directly controlled by the twistable throttle grip. The linear vertical displacement of this valve is equal to the linear vertical displacement of the needle.
- Part of a float valve system that regulates flow into the float bowl, the Float is usually a hollow plastic floating device inside the float bowl attached to a pivoting hinge. As the fuel level in the float bowl varies, the vertical level (angular displacement) of this float changes as well. This movement controls the opening and closing of the float needle.
- Float bowl
- Stores fuel at atmospheric vapor pressure.
- Float needle
- Part of a float valve system that regulates flow into the float bowl, the Float needle is a two force compression member which is about a centimeter if not smaller in length. It usually has a pin connection at one end and a rubber sealing element at the other end. As the float bowl moves, the float needle moves linearly. Its rubber seal blocks an opening through which fuel flows from the gas tank. Many leaky carburetor problems are due to a damaged float needle.
Carburetor size, usually measured by the width of the venturi at its smallest point, has a significant effect on the power, fuel economy, and noise level of an engine. Most stock mopeds came equipped with anywhere from a 9mm to 15mm carburetor. Smaller carburetors were equipped to increase gas mileage and limit speed. Increasing the size of a moped's carburetor can increase the rev ceiling and therefore top speed, as well as provide more power for acceleration. This can sometimes lead to sluggishness at lower engine speeds, depending on several other factors.
Carburetors work within a certain range of air speed passing through it's venturi. Increasing the diameter of the venturi lowers the air speed, allowing the carburetor to continue working well at a relatively higher engine speed. However, the wider the venturi, the greater the vacuum required for it to function. Engines equipped with over bore kits, or with increased compression ratios and larger transfer and exhaust porting are able to create this larger vacuum and benefit from carburetor's with increased venturi diameter.
Stock mopeds run well with anything from a 12mm to 16mm carburetor. 50cc kits or ported stock cylinders may see a benefit from up to a 19mm carburetor when paired with a expansion chamber exhaust pipe and matching intake manifold. 60cc, 65cc, and higher displacement kits work best with a 15mm, 19mm, 21mm, or even larger carburetor. Some drag racing engines use up to a 26mm carburetor with a 86cc Minarelli kit, although this engine would be difficult, if not impossible, to use on a street bike.