Need to know how to adjust the main jet on a dirt bike carb? Whether you’re trying to fix a jetting issue or you want more power, you’re in the right place!
In this article, I’ll show you what the main jet does, why it’s important to get it properly tuned, and then how to easily adjust it even if you’re new to dirt bike carb tuning.
How do you adjust carburetor jetting on a dirt bike?
Dirt bike carb tuning is usually fairly simple, contrary to what many people think. Like most anything, the more you practice, the easier it gets.
One of the biggest mistakes people make when re-jetting is making multiple changes at the same time. Another is starting with the wrong step.
In order to make jet tuning simple, you have to start with the right first step, and then make one change at a time.
To adjust carburetor jetting on a dirt bike you must start with the float level, and then go to the pilot jet circuit. You shouldn't need to worry about the main jet until you have the others properly tuned.
Mikuni carb vs Keihin carb
When comparing carburetor jetting, a Mikuni and Keihin carb do the same thing. They both have jet circuits to control the air and fuel mixture.
The biggest difference is their jetting chart when it comes to actual jet sizes. A Mikuni typically has larger numbers for the main jet, such as 390, while a Keihin carb uses numbers closer to 160 for the main jet.
The higher number on the Mikuni here doesn't necessarily mean they're bigger (richer) - it's just a different number system they use.
What does the main jet do?
The main jet controls the fuel mixture from about ¾-full throttle. There's some overlap under the ¾ turn throttle position, but that's where the majority of your tuning will be.
This means that you will be tuning for peak horsepower at high RPM, which is used mostly for racing, or if you just like to accelerate quickly and often.
Tuning a 2 stroke carb can help get rid of a lot of issues, such as bogging or fouling plugs. 4 stroke jetting isn't quite as sensitive, but it can still make a big difference if you spend a few minutes learning to properly do it.
Why adjusting the main jet might not help you
This is also why adjusting the main jet won't do that much if you're just a casual trail rider that's very conservative on the throttle - you never use the main jet if you're always at half throttle or less.
Before you start jetting the main jet circuit
With that said, you need to start at the first step. Before you adjust any jet circuits, you must make sure the float height is properly set.
The float affects all of your jetting, so if it's too high or too low, it's going to throw all of your jetting off.
After that, you need to get the pilot jet circuit properly tuned. That controls 0-¼ throttle openings. The needle controls the ¼-¾ throttle position range.
To make jet tuning simple
A common beginner's mistake is not properly warming up your dirt bike's engine. Air and fuel mixture is going to be different when the engine isn't up to operating temp.
This throws off the jetting, giving you an inconsistent and inaccurate result. Just make sure the engine is nice and hot before you start making any jetting adjustments.
How do I know what jets to put in my carb?
First, you have to know what size jets are already in your carb. Then, you have to test by feeling and listening to the bike to know whether you need bigger or smaller jets.
Remember to work on one jet circuit at a time. This means you'll be paying attention to and testing one area of throttle position when test riding your dirt bike.
Since the main jet roughly controls ¾-100% throttle (there's a slight overlap, but don't worry about that), that's where your throttle needs to be while riding to know if you need to adjust the main jet.
How do I tune my main jet?
You have to listen and feel what your dirt bike is doing, whether it’s a 2 stroke or 4 stroke. There will be certain symptoms that help you decide whether to go richer or leaner on your main jet.
To tune the main jet, it’s best to use a new spark plug. You want to find a straight road that you can ride in 4th or 5th gear for several seconds.
Shift into 4th or 5th gear at an RPM slightly above idle. Then roll on the throttle quickly to full throttle (your throttle tube will stop/lock).
Let the engine completely rev out and then stop when the power decreases or you hit the rev limiter. Based on what your dirt bike does, that’s how you where to go with the tuning.
What does a bigger main jet do?
A bigger main jet simply feeds the carburetor more gas, giving it a richer mixture. You need to go to a bigger size main jet when it’s running too lean at full throttle.
Signs your main jet is too small/lean:
- Bogging on full throttle acceleration
- Your dirt bike overheats while riding at high RPM
- Power goes flat quickly at high RPM
- Pre-ignition sound (pinging) under heavy-load acceleration
- Piston burns down and seizes (extreme case)
How do I know if my main jet is too rich?
A main jet that’s too rich is not a big issue if you’re a casual rider that generally rides at a low RPM. But, if you’re racing then you’re missing out on potentially big power gains and better throttle response.
Main jet too big symptoms:
- Sputtering/hesitation on full throttle acceleration
- Can’t reach max RPM/peak power easily
- Spooge out the exhaust (2-stroke dirt bikes)
- Lacking horsepower
- Can be harder to start when hot
How to adjust the main jet
The main jet is pretty easy to adjust. You don’t even have to remove the float bowl on your carburetor.
Just rotate the carb and remove the float bowl drain plug screw - gas will drain out, so turn the petcock/fuel valve off first.
Then the main jet is the brass part that you can see right there. You should be able to remove it easily with a 6mm socket or a cheap Motion Pro Main Jet Tool (Amazon). Then just replace it with a bigger or smaller main jet, according to your testing results.
How to prevent a catastrophic engine failure
Tuning the main jet is just one of many ways to make your dirt bike more reliable. I want to show you 3 more easy ways to make your dirt bike last longer without having a major breakdown. Tap here to learn more.