How to Understand Carburetor Jet Sizing

by Kelvin Hayes

The jets on a carburetor meter the amount of fuel that enters the throttle bores of the carburetor where it mixes with incoming air. If the engine shows performance problems such as stalling or sluggish acceleration, you'll need to change the jet sizing. Jets are threaded and have a small orifice through their center that varies based on the size of the jet. For example, a jet may be stamped (on its head) with the number 30. This indicates that the jet's size is 30 mm. Installing a smaller size jet will reduce fuel flow, which is ideal for high-altitude conditions where air is limited, where a larger jet size increases fuel flow for oxygen-rich, low altitudes. With this knowledge, you'll be able to understand how to correctly change the jets to regain the engine performance levels that were lost due to improper jetting.

Jet Sizing For Altitude Changes

Step 1

Remove the jets and insert a smaller/larger set based on the vehicles operating altitude. When the operating altitude of the engine will be 2000 feet higher than your normal altitude, you'll need to insert a jet one or two sizes smaller than the size currently installed in the carb, which reduces the amount of fuel entering the engine to match the reduced level of oxygen in the air. If the altitude is dropped 2000 feet, you'll need to insert a jet that is one to two sizes larger than the current size, increasing the fuel flow to match the higher oxygen levels. The overall goal is to maintain the correct air/fuel mixture ratio for your specific engine. An incorrect ratio will show in decreased engine performance.

Step 2

Unscrew the fuel bowl bolts with a wrench and pull the fuel bowl off the carburetor. Save the gasket that was pressed between fuel bowl and the carburetor's metering block (what the fuel bowl is bolted to). With the bowl off, the jet(s) are exposed and can be identified by their brass color and center orifice.

Step 3

Unscrew the jet(s) with a screwdriver or pair of pliers and note the size stamped on the jet's head. As mentioned in step 1, you'll need to increase the jet size for lower altitudes, while a smaller jet size will be needed for higher altitudes. Make the size change in increments of one to two sizes at a time. Screw the new jet size into the jet mounts within the metering block, with a screwdriver or pair of pliers.

Step 4

Press the fuel bowl gasket and fuel bowl against the metering block and tighten the bowl bolts with a wrench. Turn the engine on and test it by listening for a smooth idle and a crisp throttle response. If the carb continues to under perform, remove the bowl and make another one to two jet sizing adjustment, either up or down based on your circumstances, until the engine runs well.

Jet Sizing When the Temperature Changes

Step 1

Install a smaller jet size if the ambient temperature dips below 35-degrees Fahrenheit. This creates a lean condition that runs the engine hotter, ideal for cold conditions. If the air temp raises above 105 degrees, larger jets will be needed to increase the amount of fuel entering the combustion chamber, known as a "rich" condition. Rich conditions allow the engine to operate at a cooler temp.

Step 2

Unbolt the fuel bowl bolts from the metering block of the carburetor. Pull the bowl away from the carburetor, preserving the bowl gasket. The jet(s) are now exposed and can be identified by their center orifice, screwed into the metering block.

Step 3

Unscrew the jet(s) with a screwdriver of pair of pliers and adjust the jet size up or down one size, depending on your circumstances. Screw the new jet(s) into the jet mount(s) in the metering block and press the fuel bowl gasket and bowl to the metering block.

Step 4

Tighten the bolts of the fuel bowl to complete the jet sizing change. Test the engine to ensure the jet sizing change has compensated for the temp change, indicated by the engine running smooth during idle and crisp under acceleration.

Jet Sizing As It Pertains To Air/Fuel Mix Ratio

Step 1

Change the jets whenever you increase the performance characteristics of the engine. The could be anything from a larger intake manifold, long duration cam shaft or milling the cylinders for increased displacement. In essence, when an engine's performance level is increased, more air is required to run the engine. That means more fuel will be needed to maintain a balanced air/fuel mixture ratio. This ratio is different for each engine, requiring you to refer to your engine manual, new component's install manual, or by simply testing.

Step 2

Remove the carburetor bowl, bowl gasket and jet(s) as explained in the above sections. Replace the jets with a larger jet sizing by two to three sizes. Reinstall the jet(s), fuel bowl gasket and bowl as explained in above sections.

Step 3

Refer to the air/fuel recommendations of your engine's service manual. If you decide to test the engine by experimenting with different jet sizing, always start with a larger jet size than you deem necessary. This larger jet sizing may foul your spark plugs, but that's better than burning up your pistons. If your plugs do foul, you'll simply need to reduce the jet size and continue testing until the correct jet sizing is reached.