How to Change the Jets in a Carburetor

Swapping Jets 4 Power

A carburetor delivers fuel, but how much fuel it sends into the intake of the engine depends on its calibration. Mechanical or electric fuel pumps push the fuel through jets. Change the jets in a carburetor and you can change the calibration of the fuel delivery system. A crew chief on any drag race team will tell you what jet to change to if he troubleshoots a problem.

Things You'll Need

• Carburetor (Holley)
• Jets (replacement)
• Screwdrivers (flathead 4 inches long)
• Wrench (3/8, 7/16, 1/2)
• Rags

Instructions

1. Position the carburetor on a level surface. Park in a well-lit area on a flat surface to change the jets in a carburetor that is already installed in the vehicle. If the carburetor is not installed, bring the carburetor to a well-lit work bench.

2. Unscrew the bolts that hold the fuel bowl and the metering block the body of the carburetor. Each bowl in a Holley Dominator 1250cfm carburetor has four bolts holding the fuel bowls and the metering blocks together.

3. Use a wrench to turn the bolts counter clockwise until they exit the body of the carburetor. Pull them all the way out while maintaining pressure on the fuel bowl which will hold it and the metering block in place until you pull it away from the carburetor.

4. Separate the fuel bowl and the metering block from the body of the carburetor. The fuel bowl and the metering block will come apart and you can place the fuel bowl to the side. Change the brass jets that are screwed into the metering block to change the jets in a carburetor.

5. Unscrew one or more of the brass jets in the carburetor's metering block. Select from your choice of similar sized jets and replace the ones you removed with a new one of different size. The new jet will change the calibration of the fuel flow, which will result in a performance gain if done correctly.

6. Assemble the fuel bowl and the metering block together in your hands. Bring the assembly back to the carburetor and place it in the correct position to fasten it to the carburetor. Insert the four retaining bolts and tighten. Repeat on both sides of the carburetor, exchanging jets in the same way, the same size on both sides.

Tips & Warnings

• Place rags around the base of the carburetor to collect spilled fuel. Arrange your tools correctly to have them ready when you are holding the fuel bowl and the metering block to the side of the carburetor without bolts.

• You can damage the fuel bowl, the metering block, and/or the carburetor if you do not loosen and tighten the four retaining bolts correctly. Loosen each bolt slightly before removing them one by one. Avoid creating pinch points that can create defects in the metal surfaces where all the pieces are joined.

References

• Holley Dominator Adjustment Manual .pdf
• Basic Jetting Theory and Procedure

Resources

• Mopar Muscle Magazine (Why you would change jets)