How to - 4 stroke fuel screw secrets (Pics will be available soon)
by Mike Fraser
23 June 2004

Jetting a 4 stroke can be a tedious process, especially if you don't know where to start. This may become a multi-part article because dialing in the jetting of a thumper requires a lot of know how. This week we'll explore the world of the FUEL SCREW.

The first thing you must understand when it comes to jetting a four stroke is understanding which jets control the fuel flow at a particular throttle position.

There are three main circuts in your thumper's carburetor. You have your Main jet, then the needle jet, and the Pilot/Fuel screw jet.

Each jet comes into play depending on where your throttle is positioned as you turn it to accelerate. Each circut overlaps the next by a bit. Here is how they work as a general rule.

Throttle postions : Jetting Circut
Closed to 1/4th open : Pilot Jet, which is controlled by your fuel Screw on the bottom of your carb.
1/4 - 3/4 : Needle Jet and Needle
1/2 - Full : Main Jet

The Effects of weather/temperature and jetting

At higher altitudes, the bike runs richer (so you must jet leaner). The air is less dense at higher altitudes (so you're essentially getting a smaller volume of air than you do near sea level).

At warmer temperatures, the bike runs richer (so you must jet leaner). This is because the air density is less at higher temperatures. During the summer, you typically want to use a smaller Main. During the winter, you may have to go up on the Main and you may want to raise the jet needle (lower the clip).

At high humidity or when it's wet, the bike also runs richer (so you must jet leaner).

The most sound advice is this: If it is too rich, you foul the plug, but if it is too lean, you'll burn the engine. Also remember that 20% of the work will correct 80% of the jetting and get you 90% of the engine's power. That last 10% of power requires four times as much work.

Those are the basics of jetting. Now let's get to the main topic, the fuel screw.

One of the things with riding a four stroke that can be really annoying is when you go to crack the throttle and the bike bogs for a second and hesitates in delivering the power to the ground that you need at that very moment. This is usually due to too much fuel getting into the main body of the carburetor. When the throttle is closed there is no gas being pumped into the carburetor. When you snap the throttle on if your pilot jet is too big or fuel screw is turned out too far this will cause too much fuel being dumped into your carburetor's main body and cause the notorious BOG.

The fuel screw is one of the most sensitive jetting circuts and is affected by temperature, humidity and elevation changes.

Where is the fuel screw?
The fuel screw is found on the bottom of your carb. Here is a side view of your typical four stroke carburetor.

In the next image note the arrow pointing to the opening of the fuel screw.

Next you'll see how a screwdriver bit can easily fit into the hole to make adjustments. There are plenty of little specialty tools you can buy to turn the fuel screw, but what I use is a small detactable flat head screwdriver bit. Something like the following.

Next slide your hand in from the back side of the bike. This is because with the engine running and trying to squeeze your hand into such a tight spot you'll run the risk of burning your hand on the timing belt tensor. That can get pretty hot. The tensor is circled below.

WHERE AND HOW DOES THE FUEL SCREW AFFECT PERFORMANCE?
Considering that while you are racing you're on the gas (and therefore using the carburetor's needle and main jet circuits), the fuel mixture circuit might seem to be of little importance. This is a bad assumption to make. It is very important (even though it comes into play off idle). If the fuel mixture is not 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.

ISN'T A FUEL SCREW LIKE AN AIR SCREW ON A TWO-STROKE?
No. Two-strokes have air screws. Four-strokes have fuel screws. The air screw is on the side of the carburetor and regulates the amount of air that makes it to the pilot jet. A fuel screw is located underneath the float bowl and regulates the amount of fuel that makes it to the carb's main body.

HOW DO YOU ADJUST THE FUEL SCREW?
By turning the screw in and out, it's possible to adjust how much fuel mixes with the air racing down the bypass hole. Turning it in (clockwise) will LEAN the mixture and turning it out(counter clockwise) will RICHEN the mixture.

OK, HOW DO I ADJUST THE FUEL SCREW?
Before you ever touch the fuel 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.
How fast is a fast idle? About 1600 - 1800 rpm. The best way to get a fast idle and keep it is with the idle adjustment screw. Turn the idle adjustment knob clockwise to increase your idle speed.

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.

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.

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.

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.

WHAT IF I HIT PEAK RPM AT THREE TURNS?
As you turn the fuel mixture screw out, the slow speed mixture circuit is delivering 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.

IS THE PILOT JET THE BEST FIX?
Not necessarily. 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 atten­tion 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 nee­dle taper. All can affect off-throttle performance—even with the cor­rect fuel mixture screw setting.

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.

WHAT CHANGES WOULD REQUIRE ME TO MAKE A 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).

This should answer and provides you with enough information on how to set and adjust your pilot jet circut and fuel screw so your bike runs hard, crisp, and BOG-LESS.

If you are new to four strokes a great website to provide you with information and get answers to most of your questions is www.ThumperTalk.comThis is a site dedicated to owners and riders of four strokes. Go to the discussion board and pick you brand of bike. If you have a question about it, it has probably already been asked and answered.

Til next week.....KEEP THUMPIN!

Mike