How to Jet the Carb on a YFZ450
- 1). Ensure the fuel petcock is in the off position. Loosen the air box and the intake boot clamps to allow the carburetor to be rotated so that the float bowl is facing you. Using a Phillips screwdriver, remove the four screws that hold the float bowl in place. Set the float bowl aside. Using a flathead screwdriver, remove the jets.
- 2). Determine the size of the jets currently in your carburetor. The YFZ450 vary by year. If your quad has been set up for MX racing, the jet sizes will be close to: Main Jet: 150-160 and Pilot Jet: 40-50. The number will be stamped on the jet and be visible under a magnifying glass.
- 3). Determine why you are jetting your carburetor. Also evaluate the modifications you have done to your YFZ. If you are running with the air box lid off and have done the camshaft modifications common for MX racing, then you will need to provide more fuel to your engine depending on your altitude and the weather where you are racing. If you are riding in dusty areas at less than wide open throttle with the air box lid in place, then your settings will be slightly different. Install the proper size jets. Do not over tighten.
- 4). Place the float bowl back on the carburetor and tighten the four screws. Move to the other side of the carburetor and remove the plastic top by removing the Phillips head screws that secure it. Pull out the diaphragm and the slide. Tip the slide upside down and remove the needle.
- 5). Determine which needle you will use. Different fuel needles are designed for different rates of acceleration and throttle positions. Consult with a local shop or racing mechanic if you are unfamiliar with these. Once the type of needle has been determined, also find the proper clip position. A lower position toward the carb body means more fuel at idle, less at top speed. A higher needle position means less fuel at idle and more at wide open throttle.
- 6). Slide the desired needle with the clip in the correct position back into the slide. Reinstall the slide and diaphragm into the carburetor. Replace the cap with the screws, but do not over tighten. Check the position of the idle mixture screw; this is located near the float bowl on the bottom of the carburetor. Using a small flat-head screwdriver, turn the screw to the right until it lightly sits in the carburetor body. Do not tighten. Turn the screw back the desired number of turns for your jetting profile, usually around two turns out. The farther the screw is turned out, the more fuel is delivered at low speeds. The farther it is turned in, the less fuel is available at idle.
- 7). Twist the carburetor back to its original position, and tighten the clamps securing it to the air box and the intake manifold. Start your YFZ and take it for a test ride, evaluating your results.
