Chronic MX Tech tip: How to adjust the jetting fuel screw on a four stroke dirt bike
If you own a four-stroke, you need to familiarize yourself with the Keihin FCR fuel screw. It is the key to living a long and happy life with your thumper. If you don’t know anything about your fuel screw, you are in luck. We are about to teach you all you need to know in ten easy lessons.
NUMBER ONE: WHAT DO AIR AND FUEL SCREWS DO?An air screw (on a two-stroke) and a fuel screw (on a four-stroke) are like the nostrils on a race horse. They can be adjusted to control how the incoming air and fuel are mixed. When the carburetor’s slide is closed to a quarter throttle or less, it doesn’t atomize fuel effectively. For these quarter throttle or less settings, a carburetor uses bypasses to efficiently meter fuel to the engine. These passages channel air into the pilot circuit (where it mixes with gasoline). This circuit controls how the bike runs from idle to the midrange. Both the air screw and fuel screw are tapered needles that effect the mixture of gasoline and air. Both are adjusted with a screwdriver.
NUMBER TWO: WHAT IS THE DIFFERENCE BETWEEN AN AIR SCREW AND A FUEL SCREW?There are two ways to control the amount of fuel and air metered by the pilot circuit, and they differ by engine type.
Air screw (two-stroke): The externally adjusted air screw controls the amount of air that is fed into the pilot circuit. It is adjusted by turning it with a screwdriver.
Fuel screw (four-stroke): This externally adjusted needle meters how much of the fuel and air that was mixed by the pilot circuit flows into the engine
Hide and seek: Since a fuel screw is hidden under the carb, an adjustable fuel screw can make it easier to change.
NUMBER THREE: WHY DO FOUR-STROKES HAVE FUEL SCREWS INSTEAD OF AIR SCREWS?At low rpm, four-strokes create less engine vacuum than two-strokes. It is the vacuum, created by the engine, that sucks fuel through the pilot and out of the carburetor float bowl. Carburetors with fuel mixture screws flow more fuel and have more finite tuning in low vacuum situations. They are, thus, more applicable to four-stroke engines.
NUMBER FOUR: IS IT RICHER IN OR OUT?A four-stroke’s fuel mixture screw meters gas after it has been mixed with air. Turning it out lets more fuel into the engine and richens the mixture (just the opposite of the air screw). Screwing the fuel mixture screw in leans the jetting. Do you get the reverse parallel?
NUMBER FIVE: HOW DO YOU ADJUST A FUEL MIXTURE SCREW?
Set the bike to a high idle. Turn the fuel mixture screw in until the engine almost dies. Count the turns out until the engine rpm peaks. If you turn the fuel mixture screw too far out, the exhaust note will become dull and lumpy.
NUMBER SIX: WHAT DOES LUMPY AND DULL SOUND LIKE?
It is the opposite of crisp. When engine rpm peaks, the exhaust note is sharp and crisp, but if you go too far, the exhaust note will become choked and will lose that snappy sound.
NUMBER SEVEN: DOES A FUEL SCREW ADJUSTMENT EVER CHANGE?
Constantly. In morning practice, the air is cool. With more oxygen available in cold air, a fuel screw would probably work best at a quarter turn richer. Remember, richer is a 1/4 turn out on the fuel mixture screw. As the sun breaks, you’ll want to set the air screw back to its original setting.
NUMBER EIGHT: WHAT ELSE WILL CHANGE THE ADJUSTMENT?
Altitude: Elevation changes below 4000 feet have little effect on the pilot circuit.
Humidity: In humid weather, there is a chance you’ll have to lean the fuel screw setting in the afternoon as the climate dries
Rain storms: A fast approaching storm will require a richer setting.
The factory teams use temperature, humidity and histogram meters to help fine tune for each day’s conditions.
NUMBER NINE: CAN YOU FEEL THE FUEL SCREW ON THE TRACK?
If your bike stumbles off idle when the throttle is cracked-especially after you’ve been coasting off throttle-the fuel mixture screw setting is too rich. Turn the fuel mixture screw in. If the setting is too lean, the bike will hesitate, go “waaah” and have an airy feeling right off throttle. A bike that pops also hints at a lean pilot circuit. If your four-stroke pops at the crack of the throttle, suspect the fuel mixture adjustment but also check for an exhaust leak at the head pipe or slip fit.
NUMBER TEN: DOES IT REQUIRE ANYTHING SPECIAL?
Four-stroke owners should invest in a fuel mixture screw screwdriver. Since the fuel mixture screw is hidden under the float bowl, it is very hard to adjust without a very short screwdriver. Your dealer can get you one from Motion Pro. Even better, Zip-Ty racing offers an adjustable fuel screw that can be turned by hand.