Need to know how to adjust the needle on a dirt bike carburetor, or just want to know what it does?
In this article I’ll show you what the needle does, why it’s important to get it properly adjusted, and how to easily do it so your dirt bike runs better.
What does the needle do in a carburetor?
The needle controls the fuel mixture mostly from ¼-¾ throttle position. I say “mostly” because there’s slight overlap below and above those throttle positions.
So, when you accelerate at 50% throttle, the needle is affecting how your engine runs more than any other jet circuit.
How does the needle actually work?
The needle is a long, tapered rod that goes into the needle jet circuit going down to the float bowl. As you open the throttle slide, the needle let’s more fuel through.
This is because the needle has a smaller diameter at the bottom. When the needle comes up with the throttle slide, the opening gets bigger, which feeds it more fuel.
Why do you need to adjust the needle?
The needle jet circuit is extremely important to get a smooth and predictable throttle response. It also has a big effect on gas mileage, as well as torque and horsepower.
Since many of us ride at ½ throttle or less, it’s one of the most important parts of properly jetting your dirt bike, whether it’s 2 stroke or 4 stroke.
What does raising or lowering the needle do?
Raising the needle makes it run richer, while lowering it makes it run leaner. This is because you’re changing where the tapered end starts.
If the needle is higher, then the spot covering the needle jet hole is going to be smaller, which allows more fuel by and makes it richer. Lowering it essentially makes the gap smaller, resulting in a leaner jetting mixture.
What does moving the needle clip do?
The clip is simply the part that controls the height of the needle in your dirt bike carburetor. Moving the needle clip from one slot to another is what raises or lowers the needle.
Since the needle clip is very small, you need to pay careful attention when removing and installing it back on the needle. I like to use a small pair of pliers when adjusting the clip position.
Does needle position affect idle?
The needle position doesn’t directly affect the idle speed. With that said, in extreme cases it may have some effect.
If you need to adjust the idle speed, you need to adjust the air screw (2 stroke) or fuel screw (4 stroke) and the pilot jet circuit first.
What happens if the float level is too high?
Before you adjust any jet circuits, the float level needs to be properly set. If the float height is too high then all of your jetting will be rich.
When the level is too high, then more fuel is in the float bowl. This makes it easier for the jets to suck up more fuel, giving you the richer mixture.
Does the main jet affect the needle?
Yes, changing the main jet in your carb can have some affect on the needle. There is some overlap on where the fuel mixture is coming from based on your throttle position.
With that said, every bike is a little different in how it reacts to jetting changes. You also still need to adjust each jet circuit individually.
Trying to change the needle and main jet at the same time may or may not help, but you certainly won’t know which one made the most difference. To make things simple and prevent frustration, always make one change at a time.
What’s the best carburetor needle position?
The best needle position is when your dirt bike runs the best. Okay, all kidding aside – since every dirt bike is a little bit different, you won’t know what the best position is until you try a different position.
For example, the standard needle clip position is the middle slot (there’s generally 5 total clip positions). But your bike may run the best with the clip at the 2nd from the bottom position.
It’s not that hard to change it, and you can always go back to where it was. By taking a few minutes to test a different needle clip position, you may just find out how to get more power and a better running dirt bike – for FREE!
How to adjust the needle in a dirt bike carb
You have to remove the top cap on the carb to remove the throttle slide. Most dirt bikes have enough room to loosen and rotate your carb while still on the bike.
Once you get the top cap off (either screws off or you have to remove multiple screws), then carefully take the slide out. The needle is in the slide, so you have to compress and remove the spring to access the needle.
Then just pull the needle out. You can now either adjust the needle clip position or replace the needle with another one.
Refer to your service manual to view a needle jetting chart if you need to go richer or leaner than the clip position allows.
Simple ways to prevent a major engine failure
Properly jetting your dirt bike is one of the best ways to make it more reliable. There are other simple ways to prevent a catastrophic failure, but so many riders fail to do these.
I want to help you out with a free and practical cheat sheet. Tap here to download it now.