Wondering how the air screw works on your 2 stroke dirt bike carb works? This article will explain what the air screw is, why it’s important, and how to make an adjustment to it.
A 2 stroke carb is fairly simple in that it has 3 main circuits. Adjusting these circuits is commonly known as jetting or re-jetting the carburetor.
Why Is Carb Jetting Important?
It’s extremely important to have proper jetting. That is, if you want a strong running and reliable bike.
Air Screw vs Fuel Screw
It’s easy to get these mixed up. Just remember that an air screw is the adjustment screw on a 2 stroke, and the fuel screw is on a 4 stroke carb.
What Does The Air Screw Do?
The air screw is the “fine-tune” adjustment for the pilot jet (sometimes called the slow jet). The screw adjusts how much air is allowed through the pilot jet. This is important for jetting to get the proper air-fuel mixture.
On the flip side, the fuel screw on a 4 stroke determines how much fuel is allowed through the pilot jet circuit.
Why Is The Air Screw Important?
The pilot jet controls the air fuel ratio from a closed throttle (idle) to about ¼ throttle position openings.
An average rider, including myself, spends a lot of their riding from 0-¼ throttle position. This makes it extremely important to get it right to have your bike running well.
This means it’s not only important for starting, but also for trail riding. If your 2 stroke dirt bike has poor throttle response just off idle, it’s not only going to be down on power, but it’s going to be harder to control because it won’t accelerate smoothly when you give it a little gas.
When do you need to adjust the air screw?
Even if you tune it once, you’re likely going to need to adjust it again if you want your dirt bike to start and run at its best. Fortunately, it’s really easy to turn the air screw on most carburetors, so it’s a quick adjustment once you practice a few times.
You may need to adjust the air screw on your carb when:
- Temperature changes
- Humidity changes
- You ride at a lower or higher elevation
- You make a modification to your bike that affects the power
For example, if you start riding at 4000 ft. elevation in the morning at 50 degrees with your air screw properly set, then you ride up to 9000 feet elevation and it’s 75 degrees, the air/fuel mixture will need to be adjusted. Once your bike is harder to start or the throttle response is lacking, it’s time to adjust the air screw.
How To Adjust The Air Screw
The air screw is usually found near the bottom of the carb body on the left side. It’s a small screw that requires a flat blade screwdriver and is often near the fuel inlet.
Whether you have a Keihin carb or a Mikuni carb, the adjustment screw will be virtually identical when it comes to adjusting it.
Quick Tip: Always use the correct size flat blade to adjust the air screw. Using the wrong size (especially one too small) can easily damage the screw, making it very difficult to adjust.
Leaner or Richer?
How do I know whether to go richer or leaner and how do I know which way to turn the air screw? As for going leaner or richer, simply determine your carb symptoms and follow my chart below.
Symptoms of a lean 2 stroke carb air adjustment screw:
- Hard starting when engine is cold
- Bogging at smaller throttle position openings
- More likely to overheat and leak coolant out overflow tube
- Surging at low RPM
- Hanging idle/high idle
Symptoms of a rich 2 stroke carb air adjustment screw:
- Hard starting when engine is hot
- Hesitation/sputtering at smaller throttle position openings
- Spark plug fouling/wet
- Low idle/won’t stay running
Which way to turn air fuel mixture screw?
When adjusting the air screw, if you want to go leaner then you need to open it up to allow more air in. Turn it out (counterclockwise).
To make it run richer, turn it in (clockwise) to decrease the amount of air allowed through it.
Adjusting The Air Screw To Determine The Pilot Jet
I said earlier that the air screw adjustment should determine what size pilot jet should be in your bike’s carb. So how do you figure this out?
It’s actually pretty simple. The total amount of adjustment the air screw has is about 3.5 turns, based on my experience.
0 is the screw turned all the way in (clockwise). 3.5 turns is 3 and a half turns out from 0.
The goal is to get your 2 stroke bike running the best, and the goal is to do that with the air screw between 1.0 and 2.0 turns out. That way you have a little bit of adjustment both ways if you need to go leaner or richer.
A temperature or elevation change would require a quick air screw adjustment to keep your bike running crisp and strong.
When To Go Leaner vs Richer
When adjusting the air screw, if your dirt bike runs best with it 2.5 turns out or more, then you need to change the pilot jet.
Going 2.5 turns out means it wants more air. You’ll want to go to the next size smaller pilot jet to lean out the jetting circuit.
Does your bike run best at less than 1.0 turn out? This means it needs more fuel and you need to go bigger/richer on the pilot jet.
JD Jet kit vs DIY jetting
What’s the point of figuring out the jetting yourself when you can just buy a JD Jetting Jet Kit (Amazon)? The truth is, the only thing special of the JD kit is their custom tapered needles for low and high elevation riding.
Other than the needles, they just give you a standard pilot jet and main jet. These may not even be the right ones you need for you specific 2 stroke dirt bike.
You will still have to adjust the air screw because that’s built-in to your carburetor and is not adjustable.
Practical tips for when you re-jet
Jetting a 2 stroke dirt bike carburetor doesn’t have to be that difficult. Just take one step at a time so that you know how each change affects your bike.
Always remember to mark down your original settings as well as each change you make. Doing this in the beginning will help you understand how the air screw or any other jet circuit works.
Want to go more in-depth and learn how to tune any 2 stroke air-screw in just minutes? Click here to learn how.