Are you considering buying a pit bike carb upgrade but not sure if this popular mod is worth your money or what size to get? Whether you’re frustrated with your stock 125cc or bigger pit bike carb or you just want more power, you’re in the right place!
In this article, I’m going to show you:
- Why you may or may not want to upgrade your pit bike’s carburetor
- What size options you have for converting to a larger carb
- How to choose the right size carb based on your specific pit bike size and preferences
What does the carburetor actually do on your pit bike?
The carb on your pit bike or dirt bike controls the air and fuel mixture. In other words, the engine sucks air and fuel in to ignite it, but the carb determines how much air and fuel goes in, which affects the power and efficiency of the engine.
If there’s too much air and not enough gas going into the engine, then it’s going to have a lean jetting mixture, which can cause a lot of problems.
Tuning the jetting on your pit bike carb can cause or fix these symptoms:
- Starting - hot or cold
- Bogging
- Poor throttle response
- Sputtering
- Spark plug fouling
- Low power
- Less dependable/reliable
- Poor gas mileage/range
This means that if your stock or current pit bike carb is not properly jetted, then you could have significant improvement in how it starts and runs by simply adjusting one or all of the jet circuits. You can get started in just a few minutes, even if you’ve never touched a carb and are worried you might make it worse.
Why upgrade the carb?
If you already tuned your stock carburetor or are sick of dealing with troubleshooting problems, it might be worth upgrading your pit bike’s carb. For example, I did a carb swap on my TTR125's and it was a worthwhile improvement.
Going to a different size or type of carb can have a big change in how it runs, but it’s not always for the best.
When you upgrade the carb to a bigger bore size, you have the potential for making more horsepower. Going with a better/different style of carb (round vs flat-slide, for example), it can make your pit bike start and run better because it’s burning the air-fuel mixture more efficiently.
The problems with upgrading your pit bike carb
A common mistake is installing a carburetor that’s too big for your pit bike to properly handle it. Yes, going from a 24mm to a 30mm carb can give you a little more horsepower at peak RPM (8000), but it’s going to kill bottom-end power.
You see, when you have a bigger carb opening (bore size), the air velocity goes down, especially at low speeds. Think of it like the end of a garden hose - when you partially cover it with your finger, the water sprays further because the velocity was increased with a smaller opening.
What’s the goal then?
You need to figure out what your end goal is before you buy a pit bike carb upgrade - or any modification, for that matter. For example, do you want more low-end
What’s the best carb size for your pit bike?
- 24mm: Stock 125cc engine for best overall torque & HP (casual/trail riding)
- 26mm: Stock 125cc engine for best midrange & top-end HP, or best overall power for a 140-150cc modified engine (aggressive trail riding/racing)
- 28mm: Best for max HP, or better overall power for a 150cc+ modified engine (racing)
- 30mm: Best for max HP on 150cc+ modified engine (high RPM racing)
24mm carb upgrade
If you have a 125cc pit bike and want a better-performing carb that will give you better power all around, a PE 24mm carburetor (Amazon) is a good choice if you currently have a smaller carb size.
It’s a good combination for low-end torque and horsepower, especially if you have a slightly bigger or modified engine. Once you get over 24mm, you’re going to start losing major low-end torque unless you have a larger pit bike engine (150cc+).
26mm carb upgrade
Moving up to a PE 26mm (Amazon), you can expect to get even more midrange and top-end horsepower. However, you’ll lose some low-end torque and throttle response if you still have a stock 125 cc engine on your pit bike.
If you upgrade your pit bike engine to around 150cc, you can take advantage of the larger carb mod so that it gets better power at virtually every RPM.
28mm carb upgrade
28 mm is about the max size you’ll want to swap onto your pit bike, and even that might be too big unless you have a highly modified engine. So, the PE28mm carb upgrade (Amazon) is a good choice if you have a big pit bike engine (150-190cc) and want more midrange and peak horsepower.
If the carburetor is way too big for your engine to handle, it’s not only going to lose significant power at lower RPM, but it will be harder to tune the jetting because the air velocity will be slower and less consistent.
Keihin PE28mm spigot-mount carb upgrade
Maybe you have a 125-200cc pit bike or dirt bike but it has a different carb mounting system. A Keihin PE 28mm spigot mount carb (Amazon) is a good choice if you have a vertical-mount engine and want more midrange and top-end horsepower.
What pit bikes will this Keihin PE28 carb upgrade fit?
Most pit bikes have a bolt mount for the carb/intake, which will not work for the Keihin PE28 carb upgrade. This carb is made for pit bikes/dirt bikes with a vertical-style engine, such as a Yamaha TTR125 or an SSR 189.
The Keihin PE 28mm is a spigot mount carb, which means that the carb has a round spigot diameter on both ends that need to be clamped onto the airbox/filter and intake manifold, as opposed to bolting onto the intake manifold.
Jetting for your pit bike carb upgrade
There’s a chance that you might not have to adjust the carb jet circuits when you first install it - it may start easily and run fine. However, if it’s hard to start, it bogs, or just runs rough and lacks power, then you’ll have to make an adjustment - don’t worry, it’s not as hard as you think, especially on most pit bikes because the carb is easy to remove and install.
The proper steps to getting your pit bike carb adjusted
Cheap aftermarket pit bike carbs sometimes come with a float that isn’t properly adjusted, whether the assembly-line person was in a hurry or it got vibrated out of place during the delivery. The float controls all the other jet circuits, so you need to make sure the float height is properly set before making any other adjustments.
Next, you need to set the fuel screw and pilot jet for easier starting and reliable throttle response. After that, you can move to the main jet and needle clip position to fine-tune the power.
Jetting with other mods
Changing other parts on your pit bike will affect the jetting. So if you’re upgrading the engine or exhaust, this will change what jets you need compared to a stock pit bike.
How mods affect jetting
For example, a bigger engine will suck in more fuel because it’s basically a bigger pump. This will make it run richer, so you’ll need to use a leaner jetting setup in most cases.
How to easily adjust the fuel screw on your new pit bike carb upgrade
The fuel screw is extremely important if you want your pit bike to start easily and have a smooth and dependable throttle response so that your bike is easy to ride. I want to show you how simple it is to make an adjustment and how to know if what you’re doing is correct - click here to learn how.