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How To Easily Adjust Fuel Screw On A 4 Stroke Dirt Bike

Need to adjust the fuel screw on a 4 stroke dirt bike or just need to know if it will help your bike? In this article I’ll show you what the fuel mixture screw does, why you need to get it properly adjusted, and then how to easily do that.

How do you adjust a 4 stroke carburetor on a dirt bike?

There are a number of jet circuits on a 4 stroke carburetor, so how do you know where to start and how to adjust each one?

Keihin FCR Carb 2007 RMZ450 How To Easily Adjust Fuel Screw On A 4 Stroke Dirt Bike
Modern 4 stroke carb on a motocross bike

The pilot (slow) jet circuit is the first circuit that should be tuned, but before that, you need to make sure the float level is properly set. Trying to tune the jets with a float height that’s too high or low will throw your settings all off.

To easily adjust a carb on a 4 stroke dirt bike, you need to take it one step and a time and tune each jet separately. Adjusting multiple jets or screws at the same time will give you mixed or inaccurate results.

Complicated jetting made easy

If you go richer on the pilot and the main jet and your dirt bike runs better, you won’t know which jet made the change. Start at the bottom and make one small adjustment at a time.

This will turn the “black art of jetting” into a simple process. Making small changes will help you understand how each jet affects how your bike starts and runs.

Why should I tune my carb?

Making your dirt bike faster is definitely not the only reason why you should spend a little bit of time learning how to jet your 4 stroke dirt bike. There are huge advantages to properly jetting a carburetor, and there’s also huge disadvantages to NOT tuning your carburetor.

The jetting on your dirt bike carb affects:

  • How easily it starts – hot & cold
  • Throttle response – smooth or jerky
  • Horsepower & torque
  • Reliability – how long your engine will last
  • How easy or hard your bike is to ride
  • Fouling spark plugs – can leave you stranded
  • Gas mileage – how far you can ride on a tank
  • And more…
20180802 105544 How To Easily Adjust Fuel Screw On A 4 Stroke Dirt Bike
Riding technical terrain with a poorly tuned carb would make it twice as hard

What does the fuel screw do on a dirt bike?

The fuel screw controls how much fuel goes into the pilot jet circuit of the carburetor. You can change the size of the pilot jet, but the fuel screw is the “fine adjuster” to get the best performance.

For example, you may have the “right size pilot jet”, but if the fuel screw is not properly set then it will not be giving you the best performance and can cause issues.

Factors that affect fuel screw tuning:

  • The bike/engine
  • Engine compression
  • Air temperature
  • Engine temp
  • Humidity
  • Elevation
  • Air pressure

Fuel screw vs air screw on a dirt bike carb

The biggest difference is that a fuel screw controls how much fuel goes through the pilot jet while an air screw controls how much air goes through the pilot jet.

They’re essentially doing the same thing and giving you the same result. One simply adjusts the air and the other adjusts the fuel.

This also means that you have to keep in mind which you are tuning so that you know if you’re going richer or leaner. A 4 stroke dirt bike carb has a fuel screw and a 2 stroke dirt bike carb has an air screw.

So if you’re tuning a 4 stroke carb, then opening the fuel screw lets more fuel in to make it richer.

How will adjusting the fuel screw improve your dirt bike?

As I mentioned earlier, learning how 4 stroke dirt bike carb tuning works can really improve how your bike runs and rides. Adjusting the fuel screw can make your 4 stroke bike:

  • Start easier – cold or hot
  • Better throttle response – smoother & crisper
  • Increase power
  • Improve reliability
  • Increase gas mileage to ride longer on a tank
  • Prevent spark plug fouling

How do you adjust a fuel mixture screw?

Before you start tuning the fuel mixture screw or any jetting, make sure to properly warm the engine up to operating temp. A cold or partially warm engine will give you inconsistent/inaccurate jetting results.

The fuel mixture screw on a 4 stroke dirt bike is usually located on the side of the carb in front of the slide (the front half). However, it may by on the bottom of the carb body as pictured below.

To adjust the fuel screw on a 4 stroke dirt bike you must locate it first. On this carb it's on the bottom of the body.

It’s a small, brass screw that can be easily turned with a flat-blade screwdriver. You may need a very short bit or driver if you want to reach it without rotating the carb while it’s on your bike.

To adjust the fuel screw, simply turn the screw in (clockwise) or out (counterclockwise). Turning it in reduces the fuel, which makes it leaner. Turning the fuel screw out increases the fuel, making it richer.

Turning it just ⅛ of a turn can make a difference in how the bike starts and responds, so make one little change at a time. If you go richer and it keeps getting better then keep testing until the bike runs worse.

A proper fuel screw setting should be in between 1.5-2.5 turns out. Zero turns out means you have the screw fully turned in.

If the bike runs best at 2.5 or more turns out, then you need to go with a richer (bigger) pilot jet. If it runs best with the fuel screw at less than 1.5 turns out then you need a leaner (smaller) pilot jet.

How do you adjust the idle on a 4 stroke dirt bike?

Since the fuel screw and pilot jet affect the idle, it’s best to adjust the idle after tuning these. To adjust the idle, simply turn the idle screw in or out until the engine stays running at a smooth RPM.

Jetting makes your dirt bike last longer before needing a rebuild

Not only will your dirt bike start and run better when you adjust the fuel screw and jetting, but it will last longer so that you don’t need to rebuild as often.

Why? Because when the fuel mixture is too rich or too lean, it’s putting more stress on the engine parts. This wears them out quicker and could cause a major engine failure if it’s on the extreme end.

By adjusting the jet circuits for your specific bike and set up, the more efficient fuel mixture will maximize the reliability so that you can get more hours on the engine before it needs a rebuild.

Practical tips for when you re-jet

Jetting a 4 stroke dirt bike carburetor doesn’t have to be difficult. You won’t get easily confused or overwhelmed if you take one step at a time so that you know how each change affects your bike.

Always remember to write your original settings down, in addition to each change you make. Doing this at the start will help you understand how the fuel screw and jetting circuits work. 
Want to go more in-depth and learn how to easily adjust your 4 stroke fuel mixture screw in just a few minutes? Click here to learn how.