How to Rejet a Carburetor | BPNW Info Center

Rejetting a Carburetor

Introduction:

This guide will instruct you on how to change the main jet in the carburetor on your buggy to balance the air/fuel mixture and improve performance!

When changing to a higher airflow intake like a UNI filter, gutting your muffler, or upgrading to a new exhaust altogether, you will need to rejet your carburetor to a larger jet size. See the Buggy Parts NW Info Center article on Air Flow and Exhaust and/or guide to installing a UNI filter for more information.

This how-to guide works for both 150cc and 250cc engines.

Tools:
– 2 Screwdrivers (philips and flathead).

Part I: Accessing the Carburetor

1. Turn off the fuel supply to the carburetor by turning the petcock at the gas tank or pinching off the fuel line. Fuel must not be flowing during this process.

Safety Warning: Fuel will spill in the next few steps. Ensure the buggy is parked in a location where it is safe to spill gasoline.

2. Disconnect the fuel line from the carburetor.

Note: When disconnecting multiple hoses from the carburetor, make sure to mark them with a piece of tape or colored tie wrap to identify them for easier reattachment. It is vitally important to replace hoses to the same location that they were removed from. Carburetors vary in number and location placement of hoses, so this guide cannot be specific in that regard.

3. Loosen the screws on the stock airbox or UNI filter and intake manifold clamps connected to the carburetor.

4. Detach stock airbox or UNI Filter.

5. Turn the carburetor 90° in the intake manifold coupler so that the bottom of the carburetor is facing you.

Note: Some buggy models may require you to remove the carburetor completely to service it. If this is the case, take care to carefully label all hoses and wires that you need to remove to ensure that they are reattached in the proper locations.

Image 2: Turn carburetor so bottom is facing you

6. Loosen and remove the screws on the bottom part of the carburetor (the “bowl”).

Note: Depending on the brand of carburetor, there may be two to four screws holding the bowl on the carburetor.

7. Remove the carburetor bowl and bowl gasket. Be careful to not damage or lose the gasket. Undrained gas may spill out of the bowl.

Part II: Changing the Main Jet Screw

8. Inside the carburetor bowl you will see the floats and the main jet screw in the center. Using a flathead screwdriver, remove the main jet screw. Make sure the the brass base does not come loose when removing the main jet screw.

9. Install the new main jet screw.

10. Repeat steps 1 through 7 in reverse order to put the carburetor back together. Make sure the gasket on the carburetor bowl is properly seated when reattaching, and that all removed hoses are replaced to their correct locations.

Part III: Testing

Now that everything is back together again, it’s time to test your buggy.

11. Ensure that the fuel petcock has been turned back on and that gas is flowing to the carburetor.

12. Start the buggy up and let it warm up for a minute or two.

13. Rev the engine while in neutral and listen for bogging, backfiring, or other signs of engine struggling throughout acceleration. If the motor revs smoothly from idle to wide open when “flooring it”, you’re probably in good shape, but we’re going to test it anyway!

14. Spray the intake manifold area of the carburetor with carb cleaner and listen for a change in the idle. If the idle speed changes, this indicates an air leak you will need to track down and seal or repair.

Note: Be sure to not spray carb cleaner too close to a UNI air intake as the spray will go through the foam filter and give you a false “leak” signal.

Spark Plug Testing for Proper Fuel/Air Mixture

15. Replace your current spark plug with a fresh spark plug.

Note: Steps 15-18 are especially important if your buggy is not running smoothly after changing the jet.

16. Start the buggy and drive it for 5-10 minutes. Drive at a range of speeds, but try to get wide open throttle if you can.

17. Shut off the buggy and remove the spark plug. Check the color of the tip:

– If the color is very light or light brown and the engine is not running smooth you have a lean condition (too much air, not enough fuel). Check the intake manifold again with carb cleaner as in step 14. If no leaks, you will need a larger sized jet.

Spark plug close-up images courtesy BuggyMaster from BuggyMasters.com. Used by permission.

– If the color is very dark black, and the engine is not running smooth you have a too rich condition (too much fuel, not enough air). You will need a smaller sized jet.

– If the color is a dark brown or slightly black ashy color and the motor runs smoothly throughout acceleration, then you’ve got the proper fuel/air mix!

18. Slight adjustments to the fuel/air mix can be performed by adjusting the mix screw on the side of the carburetor. If you’re not getting satisfactory results from adjusting this screw, go to the next larger or smaller jet size as determined by the spark plug testing.