Carburators know how and know what.

What is Carburetor?

That's why gasoline engines are designed to take in exactly the right amount of air so the fuel burns properly, whether the engine is starting from cold or running hot at top speed. Getting the fuel-air mixture just right is the job of a clever mechanical gadget called a carburetor: a tube that allows air and fuel into the engine through valves, mixing them together in different amounts to suit a wide range of different driving conditions. Carburetors have been around since the late 19th century when they were first developed by automobile pioneer (and Mercedes founder) Karl Benz (1844–1929).

Basic Parts, functions and Tuning

MAIN JET

Function: The main jet controls the amount of fuel that’s used during half to full—throttle applications. The number on the side of the jet is indicative of its hole diameter. The higher the number on the main jet, the larger the diameter of the hole that’s running through it, and therefore more fuel is allowed to pass.

Scenarios: If you notice that your bike is cutting out or popping from about half to full throttle, then the main jet is most likely too small, and therefore not allowing enough fuel into the system (i.e. it’s running too lean). In this situation you’ll want to go with a larger main jet to allow more fuel to pass. If the main jet is too big, on the other hand, too much fuel will pass and the bike will run rich. When this is the case you may experience a lot of smoking and the bike will feel stuffy and unable to clean itself out.

Location: The main jet is located on the bottom of the carburetor underneath the 17mm nut.

PILOT JET

Function: The pilot jet controls the amount of fuel that’s allowed to pass during closed to quarter—throttle applications. Just like the main jet, the number on the side of the jet is indicative of its hole diameter. The higher the number on the pilot jet, the larger the diameter of the hole that’s running through it, and therefore more fuel is allowed to pass.

Scenarios: If your bike is feeling zingy and poppy off the bottom, which is indicative of running too lean, you’ll want to go to a larger pilot jet to give the system more fuel. If you’re having trouble starting your bike, a lot of times this problem can be remedied by simply installing a bigger pilot jet. Coersely, if you’re fouling a lot of spark plugs, the system is too rich and you’ll want to go with a smaller pilot jet.

Location: The pilot jet is located in the float bowl underneath the float bowl cover on the bottom of the carburetor.

Remove and Replace: T o get to the pilot jet you’ll need a Phillips screwdriver to first remove the float bowl. Once inside, the pilot jet can be removed using a small flat blade screwdriver. Replace the jet with the appropriate size. As with the main jet, ensure that the new jet is clean and unclogged by spraying carburetor cleaner through it. Refer to your owner’s manual to find out what size jet you’re going to need. Again, I recommend going one step at a time.

NEEDLE JET

Function: Like the main and pilot jets, the needle is a fuel metering system. It affects from quarter to three quarter throttle—basically anywhere in the mid-range. The needle takes on a new position in the carburetor by simply moving its clip position. The needle clip position is always counted from the top down, with the top slot assigned as position one.

Scenarios: If your bike is bogging down or smoking a lot you’re going to want to raise the clip position, which in turn allows the needle to drop farther down into its slot. This restricts the amount of fuel that’s allowed in, making the mixture leaner. If the bike is vibrating in your hands and really zingy, it’s probably running too lean. To combat this you’ll want to lower the clip position, raising the needle, thereby allowing more fuel into the mixture. If you’re having to run your bike with the needle at either the very top or very bottom positions, chances are good that you need to go with a different taper of needle altogether. Refer to your owner’s manual to explore your options.

Location: The needle jet is located in the slide and can be accessed through the top of the carburetor.

Remove and Change: Using a Phillips screwdriver, remove the top cover of the carburetor. After pulling the slide out of the top of the carb, pull the spring up and remove the throttle cable from the slide. With the throttle cable disconnected, use a 6mm socket to loosen the needle so that it can be pulled out of the slide. Now it’s time to change the clip position. Be careful that the clip doesn’t pop off and fly out of sight. Work on a flat surface and try your best to shield. The clip is small and a pain in the butt to find if you lose it. Once the clip is in its new position, reinstall it by following the same steps in reverse.

AIR SCREW

Function: The airscrew’s function is simply to allow air into the carburetor, so it doesn’t directly meter fuel. It is a great tool for fine-tuning the snap and overall bottom-end throttle response of your bike.

Scenarios: Any situation that is indicative of how crisp your bike is hitting off the bottom, whether you’re exiting a slow-speed corner or your bike’s sitting on the stand, can guide you in the right airscrew setting. When your bike is acting lean, turn the airscrew in (clockwise). When it feels a bit too rich, open the screw to allow more air into the mixture.

Location: The airscrew is a flathead screw that is located on the side of the carburetor.

Adjusting: The airscrew requires a flathead screwdriver for adjustments. Start by turning the screw clockwise all the way in until it reaches its closed position. A good rule of thumb is to go all the way in and then back it out one and a half turns. From there, turn it in or out until your bike feels like it has just the right amount of snap on the bottom end. The minimum adjustment is generally about one full turn out, and the maximum that you’ll ever want to go is about two and a half turns out. If you’re anywhere out of that range, look at changing the size of the pilot jet.


FUEL SCREW (four-strokes only)

Function: The fuel screw is a very important fuel-metering device on a four-stroke. In many cases it is even more important than your pilot or main jet. Performance can be increased substantially by simply adjusting this screw.

If you notice a bog when you snap the throttle, the fuel screw is most likely set too rich. When the bike’s running too lean, instead of a bog, you’ll experience a cut out and your bike may stall on you.

Location: The fuel screw is located on the bottom front of the carburetor.

Adjusting: Adjusting the fuel screw requires either a special, short-length screwdriver, or tilting or removal of the carb. The fuel screw operates in basically the opposite way that the airscrew does. If you turn it in, instead of getting richer, it gets leaner. Turning it out allows more fuel to pass, making it richer. I recommend going in quarter turn increments to fine-tune it. Anywhere between one and two and a half turns out is usually ideal. Anything outside of that range usually indicates that a pilot jet change is required.

IDLE SCREW

Function: The function of the idle screw is simply to adjust the speed at which your bike will rev at closed throttle.

Scenarios: Most people find it ideal to adjust the idle so that their bike runs without stalling, without any help from the throttle. You also don’t want your bike super high revving, either. Find that happy medium. A lot of times you’ll find that if you’re stalling your bike in corners, you can prevent this by simply turning your idle up. If your idle’s set too high, your bike may have the tendency to creep on you in slow speed corners.

Location: The idle screw is located on the side of your carburetor, usually right above the airscrew.

Adjusting: It can be adjusted with a flathead screwdriver, or sometimes with your hand on a four-stroke carb. Start your bike up and allow it to get warmed up a bit. Turn the idle screw in for higher idle, or out for a lower idle speed.