My YFZ450 Won't Run After It Warms Up

The YZF450 is one of the largest-displacement quads manufactured by Yamaha. It's a 4-stroke, single-piston quad built for motocross racing and various off-road environments. These 4-stroke engines are known for their ability to run at very low rpm; however, changes in elevation, air-fuel mixture and/or tuning may cause the engine to run incorrectly. Learn how to troubleshoot some of the common issues that may lead to a poorly running quad, so you can get back on the trail or track quickly.

Things You'll Need

  • Wrench & socket set
  • Philips and flat-head screwdrivers
  • Air filter spray cleaner
  • Air filter oil
  • Carburetor spray cleaner
  • Elevation-rated jets (optional)
  • 3 to 4 shop rags
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Instructions

  1. Tuning

    • 1

      Start the quad and immediately locate the idle control screw on the carburetor.

    • 2

      Adjust the idle control screw on the carburetor while the quad is idling using a screwdriver or your fingers. This should be the first item to check, as you will get immediate feedback on the idle level of the quad. If the idle adjustment fails to keep the quad from stalling (without an abnormally high idle speed), then you'll need to examine the air intake box.

    • 3

      Remove the seat using socket wrenches and slide the top of the plastic air box off to reveal the air box filter. Clean the box of any loose debris like sand and dirt before removing the air filter using a clean wet rag.

    • 4

      Unscrew the wing nut that attaches the filter to the box by hand and gently remove the filter. Fill the exposed air intake with a clean rag to block debris from entering the intake area.

    • 5

      Clean the filter using a filter cleaner spray, re-apply filter oil to the exterior of the filter and re-attach it to the air intake box. A dirty air filter will restrict airflow, making idling difficult or impossible. If after cleaning the air box filter the quad refuses to idle, you'll need to remove and inspect the spark plug.

    Checking for Fouling

    • 6

      Remove the spark plug by unscrewing it from the head of the engine with an open-ended wrench.

    • 7

      While the spark plug is removed, kick the engine over a few times while holding the spark plug wire near a metallic object to ensure it's sparking adequately.

    • 8

      Inspect the spark plug for fouling (black charring on the head of the plug) that would indicate the need for a replacement plug.

    • 9

      Reattach the spark plug and firmly place the spark plug wire over the top of the plug until you hear a click. If the quad refuses to idle after these steps, you'll need to clean and inspect the carburetor.

    Inspecting the Carburetor

    • 10

      Remove the carburetor by accessing and detaching the hoses and clamps that connect the carburetor to the intake. Ensure the fuel valve on the bottom of the gas tank is turned to the off position to avoid excessive spilling. With the carburetor removed, you can complete a spray cleaning and an inspection of the components, looking for loose debris and "gunk" that have built up from normal use.

    • 11

      Refer to the quad's manual for jet and reed adjustments for your elevation. Make the jet/reed changes as recommended by the manual. Follow the manual's instruction for specific model details on locating and swapping jets/reeds.

    • 12

      Rebuild the carburetor and re-install it in the quad. Start the engine and adjust the idle screw until the engine reaches a slow, but sustainable idle.