How to Adjust the Carburetor on a Honda 500FM

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Things You'll Need

  • Inductive tachometer

  • D-Type pilot screw wrench (Honda part no. 07KMA-MN9A100)

The TRX-500FM FourTrax Foreman 4x4 was introduced in 2005 to fill the role of a large utility-type all-terrain vehicle in Honda's lineup and continues to be produced as of 2012. While the TRX-500FM was designed to handle almost any situation one might encounter on a ranch or trail, it does require regular maintenance and adjustments to keep it in peak condition. To this end, the ATV's carburetor can be adjusted to via two screws; one screw sets the engine's idle speed, while the second screw regulates the air and fuel mixture used by the engine from idle to a quarter-throttle. Carburetor adjustments should be made immediately after the carburetor has been disassembled or if the engine will not hold a steady idle.

Step 1

Park the ATV on a flat, level work surface and set the parking brake.

Step 2

Reach under the right front fender and grasp the spark plug wire protruding from the front of the engine cylinder head cover. Clamp an inductive tachometer onto the spark plug wire.

Step 3

Unlock the seat, using the release lever under the left side of the seat, then remove the seat from the ATV. Pull the recoil starter cover, the panel located directly below the right side of the seat between the front and rear fenders, off of the ATV by hand.

Step 4

Turn on the inductive tachometer. Start the engine and let it idle in place for three to five minutes to allow the engine to warm to its operating temperature. Do not make any adjustments or measurements until the engine has warmed thoroughly.

Step 5

Take note of the engine's idle speed displayed on the tachometer. Ideally, the engine should be idling steadily within a 100 rpm range of 1,400 rpm once warm.

Step 6

Twist the black idle adjustment knob, located on the right side of the carburetor counterclockwise by hand to reduce the idle speed, or clockwise to increase the idle speed. Stop when the engine is idling at 1,400 rpm.

Step 7

Turn the pilot screw, located directly at the bottom of the carburetor directly under the engine's intake flange, clockwise a quarter-turn at a time with a D-Type pilot screw wrench (Honda part no. 07KMA-MN9A100). Stop when the idle speed begins to rise slightly. Alternatively, if the idle speed does not rise after one full turn, turn the pilot screw counterclockwise a full turn, then continue to turn the screw counterclockwise a quarter-turn at a time. Stop when the idle speed begins to rise slightly.

Step 8

Reset the engine idle speed to 1,400 rpm, then stop the engine.

Step 9

Disconnect the tachometer from the spark plug wire. Push the recoil starter cover onto the frame until the mounting bosses molded into the rear of the cover are seated firmly in the rubber grommets along the ATV's frame. Re-install the seat onto the ATV.


Do not make any type of adjustments to your TRX-500FM's carburetor when the engine is cold. Cold adjustments will result in a rapid change in engine idle and response as the engine warms, possibly resulting in a loss of performance and reliability. Only adjust the carburetor once it has warmed to its operating temperature.