Motorcycle Parts Basics: The Carburetor
You may know that the carburetor is key to getting the right mix of fuel and air to your motorcycle’s engine, but do you really know how it works?
Watch as motorcycle enthusiast Matt Bochnak of How-to Motorcycle Repair breaks down how a carburetor works and helps power your motorcycle.
MATT: Hey what’s up everyone, it’s Matt from HowToMotorcycleRepair.com. In this video, we are going to talk about one basic part on a motorcycle, and that is this part right here, the carburetor, also known as “carb” for short.
MATT: The carburetor is located in between the airbox and the engine, and its main function is to mix fuel and air. It then supplies this mix to the engine for combustion, which is what makes the engine create power.
MATT: The carburetor works off of Bernoulli’s principle, which you may remember from your high school physics class. The principle states that if there is an increase in fluid speed, there is a decrease in pressure. It’s similar to tying a ping pong ball to a string and placing it near running water. The moving stream of water has lower pressure than the surrounding air, so the ping pong ball moves towards the stream of water. Carburetors work in a similar way. They have a venturi, which is a tube that starts with a large diameter and smoothly transitions down to a smaller diameter. As air passes through the carburetor venturi, it speeds up, creates a low pressure zone and, therefore, allows the engine to draw in fuel from the carburetor.
MATT: Fuel is supplied to the carburetor from the fuel tank through a fuel line. A small amount of fuel is stored inside the carburetor in the float bowl, where it has to be kept at a consistent level. Fuel level is maintained by the float, which as the name implies, floats in liquid. As the fuel level rises inside the float bowl, the float arm rises up and shuts off fuel flow. As the engine consumes fuel, the float drops and adds fuel to the bowl.
MATT: Let’s remove the float and see what controls the fuel flow into the bowl area. This part is called the float needle, and this one is the needle seat. When the float rises, it pushes up on the float needle and shuts off all fuel flow.
MATT: The carburetor also has a throttle valve slide, which is opened and closed by the throttle cable. When you twist the throttle to increase engine speed, the cable pulls open the slide and allows more air and fuel to enter the engine. This causes the engine speed to increase.
MATT: All carburetors have jets that are made of brass, which are gold in color. Jets are made of brass since they are much easier to manufacture than other materials, and brass does not corrode in fuel. These jets have very precise openings so they can deliver just the right amount of fuel to the engine. Most carburetors have three jet circuits, which deliver different amounts of fuel. The pilot jet has the smallest opening, often as small as a pinhole, and supplies fuel at small throttle positions for low engine speeds. The main jet has the largest opening, and it flows a large amount of fuel at full throttle positions. The needle covers ¼ to ¾ throttle positions and is tapered so that more fuel can be supplied as more throttle is applied.
MATT: Every carburetor is equipped with a choke to add more fuel to help start a cold engine. By lifting the choke knob, additional fuel flows into the engine. After the engine is warm, the choke can be turned off, which is typically after 30 seconds to a minute of engine run time.
MATT: As you can see, carburetors are pretty simple, although they are the most common reason why motorcycles end up in repair shops. My number one tip for you is to keep your fuel fresh so that it does not gum up all those little passages we discussed in this video.
MATT: Alright, I hope you enjoyed this video on motorcycle parts basics: the carburetor.
MATT: If you’d like to see more of my videos, head over to HowToMotorcycleRepair.com, or check out my YouTube channel, MatthewMCrepair. I’m also on Facebook and Twitter. Thanks for watching and see you next time.