How Motorcycle Carburetors Workby Laura Fonda
A carburetor is an engine part that mixes how much air and fuel are injected into the cylinder where the spark plug is located. The motor creates a vacuum that allows for air to be sucked in through the air filter. From there, it is pulled into the carburetor. This air/fuel mixture is injected into the cylinder where the spark plug is located. The spark plugs ignite the mixture, push the pistons down and spin the crankshaft, which makes the motor run. It should be noted that the number of spark plugs a motorcycle has is determined by how many cylinders are on the bike. Most dirt bikes have one cylinder while most street bikes have two or four.
How it Works
According to Paul's Honda Nighthawk Pages, "most motorcycle carburetor circuits are governed by throttle position and not by engine speed." There are five main metering circuits: the pilot jet; throttle valve; needle; main jet and choke. Essentially, when you open the throttle, it pulls the slide up allowing air into the motor. The needle lifts up off the main jet at the bottom and pulls fuel up out of the float bowl. As explained by Aaron Hochnadel, who has more than 20 years' experience in auto and motorcycle repair, the bowl in the carburetor has a float inside of it, which determines how much gas is available to run the motor. This is where fuel sits until it is pulled in by either the main jet or pilot jet. The throttle slide meter determines how much air is allowed to mix in with the fuel. The choke opens an air circuit and runs the motor at a higher rpm until it warms up. Jets are used to fine-tune the power output of the motor. The pilot jet is for the bottom one-third of the throttle opening or RPMS (revolutions per minute), while the main jet is half-throttle on up. The needle technically picks up the transition between the pilot and the main jet. For dirt bikes, different size jets are used depending on altitude, temperature and motor size. Street bikes, on the other hand, have jets that are sealed and tamper-proof due to emissions regulations.
Changing and/or cleaning the air filter is one of the best do-it-yourself maintenance items that will keep your carburetor running smoothly. A clean air filter will ensure air flow will not be restricted and dirt will not be pulled into the carburetor, which could ruin the motor. You can spray carburetor cleaner on the outside of the casing to keep linkages from gathering dirt and hindering performance. For dirt bikes another do-it-yourself item includes changing out jets to accommodate different conditions. A service manual will have jetting specifications.
- Paul's Honda Nighthawk Pages
- Aaron Hochnadel, Auto and Motorcycle Mechanic; Thornton, Colorado
- photos by Laura Fonda