The carburetor used by Yamaha's Raptor 50 all-terrain vehicle relies on a constant flow of air to draw fuel into the engine. An adjustable pilot screw controls the carburetor's pilot system, which feeds the engine from idle to a quarter throttle through a single fuel jet housed under the carburetor. The position of the screw determines how much air is pulled into the pilot system, drawing more fuel in the process. While Yamaha's service manual does reference the pilot screw, it does not mention its location or how to adjust the screw to accommodate changing riding conditions.
Things You'll Need
- 5 mm Allen wrench
- Phillips screwdriver
- 10 mm socket
- Socket wrench
- Flat-head screwdriver
- Inductive tachometer
Pull the seat release lever located under the rear fender, then lift the rear of the seat up until it can be pulled off the ATV.
Remove the front grill panel using a 5 mm Allen wrench. Pull the cover off the center of the handlebar. Unplug the neutral and oil light connectors from the bottom of the cover.
Unscrew the fuel cap, then remove the trim clips from the fuel tank cover using the tip of a Phillips screwdriver. Lift the fuel cover off the ATV. Remove the front fender using a 10 mm socket and a socket wrench.
Remove the black rubber pilot screw cover from the left side of the carburetor, located just above and to the right of the idle adjustment screw, using the tip of a small flat-head screwdriver.
Turn the pilot screw clockwise a half-turn at a time, using the flat-head screwdriver, until it lightly seats against the carburetor. Count the number of turns it took to seat the screw and write it down in your Raptor 50's owner's manual for future reference; this is the factory pilot screw setting. Turn the pilot screw counterclockwise the same number of turns needed to seat the screw.
Start the ATV and let it idle in place for three minutes to warm the engine to its operating temperature. Turn the pilot screw clockwise a quarter turn at a time until the engine idle speed increases. If the engine idle speed does not increase, reset the pilot screw, then turn the screw counterclockwise until the engine idle speed increases. Do not turn the screw more than three counterclockwise turns. Turn the pilot screw back a quarter-turn, once the engine idle speed increases in either direction.
Reset the pilot screw to the factory setting if the engine idle speed does not increase in either direction. This means the factory pilot screw setting is appropriate for the riding conditions and any modifications made to your ATV.
Turn the idle adjustment knob counterclockwise one-eighth of a turn at a time to return the engine's idle speed to its normal setting -- approximately 1,750 to 1,850 rpm. Check the engine idle speed using an inductive tachometer clamped to the spark plug cable.
Stop the engine and push the rubber cap over the pilot screw once the pilot screw and engine idle speed have been adjusted. Reinstall the front fender, fuel tank cover, front grill panel and seat onto the ATV. Start the ATV and let it idle for three minutes to fully warm the engine, then take the ATV for a test ride. The ATV should idle smoothly and accelerate from a complete stop without hesitation. Make further adjustments to the pilot screw if the ATV hesitates while accelerating from a complete stop.
Tips & Warnings
- Don't turn the pilot screw farther than three counterclockwise turns. Doing so loosens the screw enough to vibrate loose while riding the ATV.
- Do not smoke or work near an open flame while servicing your Raptor 50's carburetor.
- Yamaha YFM50S Service Manual; Yamaha Motor Corporation
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