At up to a quarter-turn throttle, your four-stroke's carburetor slide doesn't process enough oxygen to effectively atomize fuel through the main circuit. To keep a four-stroke engine from stumbling all over itself, engineers designed a separate carburetor inside the main body. This secondary carburetor has its own air and fuel circuit and controls how the bike runs off idle and on into the midrange.
Your carb's secret carb is called the fuel mixture circuit, and here are the answers to every question about it.
Univac: Unlike a two-stroke carb, which is little more than bathroom plumbing with a flapper valve, a four-stroke's carburetor has more bells, whistles and gizmos than a one-man band. Luckily, the only thing you need to adjust on a regular basis is the fuel screw (located under the float bowl).
Special tool: To adjust the fuel screw on a YZ-F, CRF or KTM-SX, you need a special screwdriver. Why? Because space between the bottom of the float bowl and the tranny is in short supply. Motion Pro and Pro-Tec sell the mini-drivers. Zip-Ty Racing makes an adjustable fuel screw that can be turned by hand.
QUESTION ONE: WHEN DOES THE FUEL SCREW COME INTO PLAY?
Considering that most of racing takes place while you're on the gas (and thus on the carburetor's needle and main jet circuits), the fuel mixture circuit might seem to be of little importance. Au contraire.
It is very important (even though it comes into play off idle). If the fuel mixture is mis-adjusted, your bike will run rough and stumble across the whole range. The adjustment of this circuit is so crucial that there are both external and internal ways to tune it.
QUESTION TWO: WHERE DO I FIND THE SECONDARY BYPASS?
Under the slide. The secondary carburetor starts at that little hole pointing straight back from the bottom behind the slide. These holes channel air into the fuel mixture circuit where it is mixed with gas. This circuit controls how the bike runs off idle and into the midrange. It's adjusted by the fuel mixture screw�a tapered needle that opens or closes the flow of mixed air and gas.
QUESTION THREE: HOW DO YOU ADJUST THE FUEL SCREW?
You turn it. By turning the screw in and out, it's possible to adjust how much fuel mixes with the air racing down the bypass hole.
QUESTION FOUR: ISN'T A FUEL SCREW A GLORIFIED AIR SCREW?
No. Two-strokes have air screws. Four-strokes have fuel screws. The air screw is on the side of the carburetor and meters the amount of air that makes it to the pilot jet nozzle. A fuel screw is located underneath the float bowl and meters the amount of fuel that makes it to the carb's main body.
QUESTION FIVE: WHY DO FOURSTROKES USE A FUEL MIXTURE SCREW?
Because they don't have enough low-speed vacuum for an air screw. At low rpm, four-strokes create less engine vacuum than two-strokes. It's the vacuum created by the engine that sucks fuel out of the float bowl and into the engine. Since a fourstroke doesn't have as much vacuum, it can't draw enough air to make the mixed fuel flow well enough to keep a thumper running. By metering fuel, which is easier to draw through the orifices with low vacuum pressure, a four-stroke is able to start more easily and run cleanly at its very low rpm idle speed.
QUESTION SIX: ISN'T THERE AN AIR/FUEL SCREW?
No. That is just another name for the fuel screw. Forget it! It's an air screw on a two-stroke and fuel screw on a four-stroke. Don't mix metaphors when mixing fuel and air.
QUESTION SEVEN: WHY DON'T THEY CALL IT A GAS MIXTURE SCREW?
Fuel is not gasoline. It is the combi�nation of gasoline and air mixed together. Gasoline, while highly volatile, couldn't be a fuel without air.
QUESTION EIGHT: HOW CAN I TELL A FUEL SCREW FROM AN AIR SCREW?
It is possible for a four-stroke to have an air screw. Many vintage thumpers used two-stroke carbs� and thus have air screws. Here is how you can tell if the carburetor has a fuel mixture or air mixture screw. (1)
Look at the carburetor from the side that has the mixture screw on it. (2)
Draw an imaginary line down the carb's slide to visually split the carburetor into two. (3)
If the screw is located on the airbox side of the carburetor, it is an air screw. (4)
If the screw is situated on the engine side of the carburetor, it is a fuel screw.
QUESTION NINE: ARE THEY ADJUSTED THE SAME WAY?
No. An air screw meters air before it reaches the pilot jet. Turning an air screw in restricts the air flow and richens the mixture (turning it out leans the mixture by letting more air in). Conversely, a fuel mixture screw meters gas after it has been mixed with air, thus it works the opposite of an air screw. Turning the fuel screw out lets more fuel into the engine and creates a richer mixture. Turning the fuel screw in limits the flow of gasoline and leans the jetting.
QUESTION TEN: HOW DO I REACH THE FUEL SCREW?
The under-the-float-bowl location of the fuel screw makes it next to impossible to reach. It takes a special screwdriver (Pro-Tec and Motion Pro offer them). Ty Davis' Zip-Ty Racing offers an adjustable fuel screw that can be turned by hand (without the use of tools).
QUESTION 11: HOW DO I ADJUST THE FUEL SCREW?
Not so fast. Before you ever touch the fuel mixture screw, you must first warm the engine up. That means riding the bike around a bit, not just starting it up. Once the bike is warm, you need to bring it up to a fast idle.
QUESTION 12: HOW FAST IS A FAST IDLE?
How fast is a fast idle? As a rule of thumb, it is about 1800 rpm. The best way to get a fast idle and keep it is with the idle adjustment screw.
Some race mechanics simply crack the throttle by hand and hold it there. This is common, but it's much more accurate to set the fast idle with the slide stop.
QUESTION 13: WHAT IS THE NEXT STEP?
With the bike holding a steady, high idle, use the adjustment tool to slowly screw the fuel mixture screw in. Continue tightening the screw until the engine rpm drops (and nearly dies). Now, slowly turn the screw back out. You will hear the engine rpm begin to speed up and the exhaust note will become crisper.
QUESTION 14: WHEN DO I STOP TURNING THE SCREW OUT?
Stop turning the fuel mixture screw at the exact moment when the engine hits peak rpm. Peak rpm is when the engine runs the cleanest and fastest. When you reach that point, the idle won't become faster the more you turn the fuel mixture screw out. If you keep turning the screw, the rpm will stay up but the exhaust note will become dull, flat and lumpy.
QUESTION 15: WHAT'S A FLAT. DULL AND LUMPY EXHAUST NOTE?
It's not defined by speed but rather by crispness. When engine rpm just peaks, the exhaust note is sharp and crisp. As you continue to turn the fuel mixture screw, the exhaust note will become choked by too much fuel and will lose that snappy sound.
QUESTION 16: BUT WHEN DO I KNOW TO STOP?
Once you've trained your ear to hear what peak rpm sounds like� and have a good idea of the point where turning the screw out farther hurts performance�you might test your setting by turning the fuel screw in the opposite direction (until the rpm drops). Stop at that instant and turn it back in again. If you're not 100 percent certain you have peak idle, start counting the turns out from the spot you think is peak idle. If the engine rpm doesn't pick up after a 1/4 twist, turn it back to the original spot.
QUESTION 17: HOW DO I TELL WHAT MY SETTING IS?
Count the turns. With the engine off, turn the fuel mixture screw in while carefully counting in l/8th turn increments. Keep counting until the screw lightly bottoms. Now, turn it out the identical number of l/8th turns. That is your fuel mixture adjustment and it's usually between 1/2 to two turns out.
QUESTION 18: WHAT IF I HIT PEAK RPM AT THREE TURNS?
As you turn the fuel mixture screw out, the slow speed mixture circuit is metering more and more fuel to the engine. If the engine doesn't reach peak rpm until the screw has been turned more than two turns, it might indicate that the pilot jet is too small (lean). Try the next larger pilot and retest the fuel mixture adjustment. Conversely, if peak rpm is reached before you turn the fuel mixture screw out a half turn, it could mean that the pilot jet is too rich. Try the next smaller size.
QUESTION 19: IS THE PILOT JET THE BEST FIX?
No. Even if the fuel mixture screw is set at 2-1/2 turns out, the bike might not run as well with the next size richer pilot. But you still need to try it in order to rule it out. Always try a respective leaner or richer pilot if the fuel mixture screw's best setting is on either side of the one-half or two and one-half turn adjustment range. Pay atten�tion to how the bike runs at the crack of the throttle. Switch back and forth if necessary. Do not rule out the needle clip position or nee�dle taper. All can affect off-throttle performance�even with the cor�rect fuel mixture screw setting.
QUESTION 20: HOW OFTEN SHOULD THE FUEL SCREW BE CHECKED?
Every race day. Twice a day if it is cool and overcast during practice and bright and sunny for the first moto. When the track dries out and the sun breaks out, you'll want to set the mixture screw back to a leaner setting.
QUESTION 21: WHAT CHANGES NECESSITATE FUEL SCREW ADJUSTMENT?
Temperature isn't the only thing that affects the fuel screw. When racing in elevations above 4000 feet, the thin air will create the need for more air (turn the fuel mixture screw in). In humid climes, you might need to lean the setting in the afternoon as the day dries out. A fast approaching storm will require a richer setting (turn the fuel mixture screw out).