 Tuning and fiddling with the jetting of your carburetor is often seen as a complete headache, but it doesn't have to be. Stay tuned to find out why. Although carburetors are one of the most well-designed and engineered components of a dirt bike, most people don't really know how to jet a carb to their needs. First things first, let's understand the different components of a carb. From the air filter to the inlet manifold, there's the main air passage. The slide crosses the main air passage and the reduced cross section created by the slide is called the throat. This variation is responsible for accelerating the airflow going to the engine, while also creating a low pressure point on the throat and thus sucking fuel into the airflow. Between the slide and inlet manifold, there's the idle circuit, which is responsible for feeding the engine during idle and up to one-eighth of throttle through the pilot jet. On the idle circuit, there's also an adjustment screw, which allows some tuneability of the air-fuel ratio of the idle mixture. Now for the really interesting part. On the slide, there's the jet needle. This needle has a tapered shape and fits into the needle jet, which is mounted directly under the slide and connects to the float pole. When the slide is fully down, the jet needle blocks completely the fuel flow from the float pole to the main air passage. Screwed onto the bottom of the needle jet, there's the main jet, which is accessible through the float pole. Now let's understand how each of these circuits affect the throttle response. There are three tunable jetting systems on a carb. The idle jet, the needle and needle jet and the main jet. From a fully closed throttle position, the idle jet is what keeps the engine running up to around one-eighth of throttle. If we continue to open the throttle, the idle circuit starts fading out. While the slide lifts, the area between the needle and the jet needle increases, allowing more fuel to be sucked through the main jet to the main air passage. This circuit is responsible for the mid-throttle response. When the slide is high enough so that the needle is no longer blocking the needle jet, the main jet becomes the limiting system for fuel flow. This jet is responsible for the end of throttle response and full throttle. Now comes the fun part of jetting your carb to your liking. It's a lot easier than you might think. Let's say your bike is running too lean on idle or that it's stalling easily at little over idle and you have set the air adjustment screw already too much in. Changing to a bigger idle jet will do the trick. If the opposite happens, you can change to a smaller idle jet, leaning the mixture out. Now let's take a look at the mid-throttle response. If you feel the bike bogging too much, it's because the mixture is too rich and you have to lean it up. To do that, take the needle and place the circlip a groove higher. This will position the needle lower on a needle jet and it will obstruct the fuel flow for longer, leaning out the mixture. You can look at it as delaying the fuel flow from the needle and the needle jet. When the opposite happens and the mixture is too lean, place the circlip a groove lower to make the mixture richer sooner on the mid-throttle response. This can be useful if you want more power on mid-throttle applications. But let's say that every time you place the circlip on a lower groove, the engine performs better. But the needle has no more grooves left. This is where you change the needle jet. If the engine requires more fuel on mid-throttle and on full throttle and runs just fine, just install a bigger needle jet. Don't forget to reset the needle circlip in the middle position and repeat the process until you have the engine running butter smooth on mid-throttle. And what about full throttle? If you need a leaner mixture on full throttle to avoid foul plugs, change to a smaller main jet. If you need a richer mixture to have a cooler engine at full throttle, for example, change to a bigger main jet. Most of us won't be tuning a carb from scratch. We'll be doing some fine tuning to an extensively tested carb. This means that the base setting will be a good starting point. However, the most curious home tuners should have a question in mind. How can I change a specific area on the mid-throttle response without changing the whole thing? The answer is the needle. What if you just want to make it richer on the higher part of the mid-throttle response? Just install a needle with a thinner tip. On that throttle range, the needle will allow more fuel through the needle jet, making the mixture richer. Changing the tapered shape of the needle will have a huge impact on the mid-throttle fuel flow. Different shaped needles will provide different engine responses and it all comes down to how you want the mixture to be for a specific application. I hope you found this video useful and if you have any questions, leave them on the comments below. Thank you for watching and don't forget to subscribe.