How Motorcycle Carburetors Work an Introductory Guide

If you’re wondering how does a carburetor work on a motorcycle, or how carburetors work in general, you’ve come to the right place. The carburetor is a mechanical device which mixes air and fuel in the correct ratios. It then provides this calibrated mixture over the operating range of the engine. The carburetor has to make these adjustments based on the throttle input from the rider.

Butterfly Valve Carburetor for a two stroke engine.

The carburetor’s operation, discussed in this article, is rather easy to understand, but tuning a carburetor can be a much more difficult process. Due to increased emission standards, performance requirements and efficiency motorcycles have shifted to fuel injection. Although carburetor’s have been replaced by fuel injection on most newer engines, there are still plenty of motorcycles that utilize carburetors.

Carburetor Basics

The carburetor functions on the principal that as fluid velocity increases, the fluid’s pressure decreases. In this case, air is the fluid. This discovery was found by Giovanni Venturi in 1797. He was able to come across this finding by using a tube which had an area restriction. The fluids velocity would increase in the restriction of the tube.

In the carburetor, the air tube (venturi) gets smaller increasing air speed and dropping pressure.

So how does this relate to a carburetor? Well this discovery is important because if you connect a second tube full of fluid (gasoline) to the restriction in the first tube, the gasoline will get sucked up into the air stream. This is basically how a carb works. There’s a lot of precision engineering that goes into designing a carburetor. The air flow restriction is perfectly sized to drop the pressure, so that a specific amount of fuel can enter the air stream.

Ideally this is where the fuel is vaporized. This is possible because the increased air speed lowers the air pressure allowing fuel to vaporize. The fuel vapor and air molecules then will travel to the engine where they are ignited. This is a very important part of the combustion process. Liquid fuel is much harder to ignite than fuel vapor.

Throttle Position in Relation to The Carburetor

Did you know that the throttle isn’t directly tied to the amount of gasoline that gets to the engine? When you twist the throttle on your motorcycle what your actually doing is increasing increasing the amount of air that flows through the carburetor. This increase in air will cause a pressure differential in the venturi. The higher pressure will enable the air stream to take more fuel with it.

Twisting the throttle enables more air to enter into the venturi.

So in the end twisting the throttle increases the amount of air that gets sent through the carbs. The added air flow will suck up more fuel and send it to the engine for combustion.

Carburetor Types

There are different types of carburetors, and they are usually classified by path air enters and exits the carb when its own engine. The orientation of the carburetor along with the path of the air determine the classification.

Sidedraft: Almost all motorcycles that have carburetors are sidedraft. Sidedraft carbs are named because the air and exits enters horizontally through the tube. The fuel is sucked vertically upwards into this tube. Sidedraft carburetors are used on motorcycles usually because of size constraints on engine, and also the intake runner length to multiple cylinders can be kept close to equal. This allows equal performance and similar operating conditions between cylinders.

Updraft: An updraft carburetor intakes air at the bottom of the tube and fuel is sucked in from a horizontal orifice. The air and fuel mixture then exits from the top of the tube.

Downdraft: As you’ve probably already guessed. Downdraft carburetors work in the opposite direction of updraft. The air enters from the top of the tube. Fuel still enters in from an orifice on the side, and the air fuel mixture exits from the bottom of the tube. The benefits of a downdraft carburetor is that it is gravity assisted. The gravity will help to pull the air and fuel mixture down and out of the carburetor and to the engine.

Components of a Carburetor

Learning the individual components of the carburetor can greatly improve understanding of it and how it functions. Most carburetors have a float bowl. A float bowl is somewhat like a miniature gas tank where the fuel hangs out. This float bowl could be mounted to the side of the carburetor or more commonly the bowl is detached from the body.

Inside the float bowl is a float. The float has the same function as the float in your bathroom’s toilet. The float interacts with a needle valve which in turn controls fuel flow through the jet.

Float and needle both work to control fuel flow into the carb without overflowing the float bowl

One important thing to not is the conical nature of the needle valve’s tip. The needle valve has a fuel metering function. The conical tip is specially designed so that as the needle lifts off of its seat it increases opening orifice allowing more fuel to flow. Its important to note that these is a metering function and not an on off switch function.

From the float bowl, there are jets leading to the main body of the carburetor. These jets are usually made from brass and machined to precise diameters. The jet’s diameter is extremely important as it will determine the amount of fuel that gets sent to the carb.

Other components that exists depending on the carb include: air needles, fuel needles, or needle jets. The air needle is similar to the needle in the float but its function is obviously for air and not fuel. The needle jet looks like an actual needle, but it is much thicker.

Carb Jets and Throttle Position

Depending on the throttle position your carburetor could be relying on different jets to get its required fuel supply. Since your motorcycle carburetor jets can’t adjust their diameters, your carburetor will need a different jet for idle and for full throttle.

During idle and up to 1/4 throttle your motorcycle should be using a pilot or sometimes called idle jet. This jet is sized for fuel delivery when the throttle is closed (idle) and up to 1/4 of wide open throttle. The pilot jet is essential when starting your motorcycle as is the choke. If you don’t know what a choke is or want to learn more about it, check out this article here.

From 1/4 to 3/4 throttle the fuel flow can be controlled by the needle jet. Knowing which jet is responsible for fuel delivery at a given time can save hours of troubleshooting.

Now from 3/4 to full throttle, the fuel delivery is controlled by the main jet. The main jet will have will have the largest diameter. During this time there is still fuel being supplied through the pilot jet, but a much lower flow rate than the main jet.

One of the things you may be surprised about when taking the carburetor apart for the first time is the small size of these jets. Its not hard for these jets to become clogged or gummy overtime. This will cause fuel delivery issues and cause your bike to not run right. The good news is that a lot of these issues can be solved by yourself in your own garage.

Carburetor Location on a Motorcycle

Unlike cars, many motorcycles don’t have fuel pumps. Instead most motorcycles rely on a gravity fed system to feed the carb’s float bowl from the gas tank. This means that the gas tank is located above the carb and the carburetor is usually pretty close/underneath the gas tank.

Let us know in the comments below if you enjoyed this intro guide to carburetors, and be sure to check back for future troubleshooting content on carburetors.