Instructions on How to Set Jets on Carbs

Carburetor style varies on different engines, but the hoses are visible here.

Carburetors are the devices in an automotive engine that combine the fuel with air in the correct proportions for compression and combustion. The fuel fills a chamber in the top of the carburetor, and is sprayed into the cylinder through a narrow jet port. The ratio of fuel to air, can be adjusted by setting the rate at which the jets supply the fuel. This causes your engine to run a "leaner" or "richer" mix, affecting performance and fuel consumption.

Things You'll Need

• Vacuum pressure gauge
• Cross screwdriver or adjustable wrench

Instructions

1. Open the hood of the car, or gain access to the carburetor adjustment screws on the motorcycle. They can be found between the airbox, and the engine's cylinders. Before starting the adjustments, ensure that the exhaust pipes are clean and unclogged.

2. Remove the carburetor bodies from the engine. Depending on the configuration of your engine, you may need to remove the airbox or filter in order to access them. With the carburetor removed, clean the bowls and jets thoroughly. The narrow jet ports clog easily and can interfere with fueling.

3. Reassemble and replace the carburetors in the engine. Start the engine and get it fully warmed up. Connect the hoses from the vacuum gauge to the ports on the carburetors, and check the dials on the gauge. The idea is to make them all give the same reading, by turning the idle adjustment screw on the carburetor body. Check the user manual for your vehicle if you have trouble locating this.

4. Disconnect the vacuum gauge once all the carburetors are balanced, and reconnect the air hoses from the engine. Take the vehicle out for a run, testing the engine at various rev levels. Drive gently and hard, paying attention to the sound of the engine. Listen for misfires and check for smoke. Misfires mean the jetting is too lean, and the carburetors need to be adjusted to run a richer mixture. Smoke means that too much fuel is entering the cylinder, and burning on the piston, this requires a leaner mixture to prevent from fouling the engine.

Tips & Warnings

• It is possible to set the jets without a vacuum gauge, by simply listening for misfires as you adjust each idle screw. However, this is much harder, as it is more difficult to identify a problem cylinder from the sound of the engine.

References

• Aircooled.net; Basic Jetting Theory and Procedure; John Connolly; November 1999
• Kawtriple.com: Basic Carb Adjusting Procedures/Guidelines
• Kawtriple.com: Carb Sync Methods