In this series, I’ll be taking you through the workings of your Aprilia RS 125’s carburettor and talking you through the process of jetting your carburettor.

Introduction

Jetting is the process of adjusting the air and fuel jet sizes in order to fine tune your bike’s carburetion to suit the load demands on the engine and attempt to optimise power delivery. If you make any modifications to your engine (air box and exhaust included!) you need to know about jetting your carburettor. In this series you will learn the correct way to setup your carburettor after modifying your bike, how to tell if you are running lean or rich and what you can do about it.

In order to understand the best setup for your carburettor, you first need to understand how it works.

What parts make up the carburettor?

The purpose of the carburettor is to provide your engine with the correct ratio of fuel and air. Although the carburettor can’t provide the ideal mixture at all loads, the different parts of the carburettor work to try and create this ideal mixture for proper combustion.

The RS125 carburettor consists of the following parts:

• Fuel bowl
• Shut-off needle
• Floats
• Main jet (provides mixing above 1/2 throttle)
• Pilot jet (provides mixing from idle to 1/4 throttle)
• Jet needle (regulates flow of the main jet as the throttle is opened; 1/4 to 3/4 throttle)
• Needle jet (passage to which the jet needle resides)
• Air screw (adjusts the amount of air from idle to 1/4 throttle)
• Idle screw (adjust the idle speed)

How does the carburettor mix air and fuel?

Fuel from the tank enters the bowl through a small passage until the floats rise up enough to force the shut-off needle into the passage (prohibiting any more fuel from entering the fuel bowl). The main jet is usually found near the bottom of the fuel bowl. As the piston moves down, it draws air into the cylinder. This air flows through the throat of the carburettor and creates a vacuum which sucks fuel into the cylinder via the main jet.

The mixture screw is able to adjust the air in small amounts in order to fine tune the mixture (usually at idle). Most carburettors require adjustment of this screw and a jet change. When you change a jet, you are simply giving the fuel a smaller or bigger hole to flow through, thus changing the amount of fuel that reaches the cylinder.

What is meant by running rich or running lean?

These terms refer to the amount of petrol being delivered to the engine. If you are running lean, then the ratio of fuel to air is too low, that is, there isn’t enough fuel reaching your engine for the amount of air that is reaching the engine. This can cause the engine to run hotter and leave you with a seized piston. Running rich means that there is too much fuel reaching the engine for the amount of air that is reaching the engine. Running slightly rich is generally acceptable and protects the engine as the excess fuel helps to carry heat out of the engine. If you are running very rich, you will have a very smokey exhaust and performance will suffer.

The ideal ratio of air to fuel is called the stoichiometric ratio.

The next part of the tutorial will look at preparing your carburettor for jetting by servicing it. Remember, you can’t rush perfection!