Main Jet
Main Jets are the most critical metering system in most motorcycles. It is located at the bottom of the bowl generally near the Pilot Jet. The Main Jet helps to ensure the engine is operating under full power and prolong the engine life. The Main Jet is predominant especially in racing usage. It controls 60% to 100% of the throttle opening and provides fuel to the engine from about 1500 RPM right up to maximum RPM.
Usually, the Main Jet is the last jet you need to deal with. It is easy to get the right tune. Main Jets have different sizes of holes that determine the maximum flow rate of the fuel into the Venturi. A bigger Main Jet has a bigger hole so it lets more gas flow into the engine. The higher number of Main Jet, the richer air/fuel mixture that you will get. It is not only measures the quantity of gas into engine, it also helps in cooling the motorcycle engine.
Main Jet replacement is super easy. When you begin to adjust the Main Jet, it is advisable to start the replacement with an oversized jet. Although it will make the engine runs very rich, but it will ensure that you do not cause any damage to the engine. From there, you can gradually decrease the size (in increments of 5) of the Main Jet until your engine runs properly. Once you find the size that works well, you can make fine tune adjustments by trying different size jet around that size but using smaller increments.
Pilot Jet
Pilot Jet is also referred to as the slow jet or idle jet, since it controls the fuel delivery during start-up and idling of your bike. It runs the engine when the throttle is closed. This means Pilot Jet supplies the engine requirements of fuel and air to the engine at low RPM only. The Pilot Jet has a major influence on fuel flow from zero to 1/4 of throttle opening. When you open the throttle about one eighth of the way, all the gas / air mixtures going into the engine is controlled by the Pilot Jet.
In terms of design, Pilot Jet is a medium sized brass jet with a calibrated hole in it. It is located inside the float bowl next to the needle jet or Main Jet location. It can be accessed through the bottom of the carburetor (remove the bowl). You should check the Pilot Jet if your bike has trouble getting fuel when starting, or has a rough idle at low RPM. If your bike feels zingy off the bottom, this indicates that the mixture is too lean. This problem can be remedied by installing a bigger Pilot Jet to give the system more fuel and air mixture.
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