Motorcycle Carburetor Theory 101
Motorcycle carburetors look very complex, but with a little theory, you can tune your bike for maximum performance. All carburetors work under the basic principle of atmospheric pressure. Atmospheric pressure is a powerful force which exerts pressure on everything. It varies slightly but is generally considered to be 15 pounds per square inch (PSI). This meansthat atmospheric pressure is pressing on everything at 15 PSI. By varying the atmospheric pressure inside the engine and carburetor, we can change the pressure and make fuel and air flow through the carburetor. Atmospheric pressure will force high pressure to low pressure. As the piston on a two stroke engine goes up (or goes down on a four stroke engine), a low pressure is formed below the pistoninside the crankcase (above the piston on a four stroke). This same low pressure also causes a low pressure inside the carburetor. Since the pressure is higher outside the engine and carburetor, air will rush inside the carburetor and into the engine until the pressure is equalized. The moving air going through the carburetor will pick up fuel and the fuel will mix with the air. Inside a carburetoris a venturi, fig 1. The venturi is a restriction inside the carburetor that forces air to speed up to get through. A river that suddenly narrows can be used to illustrate what happens inside a carburetor. The water in the river speeds up as it gets near the narrowed shores and will get faster if the river narrows even more. The same thing happens inside the carburetor. The air that is speedingup will cause atmospheric pressure to drop inside the carburetor. The faster the air moves, the lower the pressure inside the carburetor. By placing tubes inside the venturi, we can use this lower pressure to put fuel into the air stream. Most motorcycle carburetor circuits are governed by throttle position and not by engine speed. There are five main metering systems inside most motorcyclecarburetors. These metering circuits overlap each other and they are: * pilot circuit * throttle valve * needle jet and jet needle * main jet * choke circuit

Fig 1 The restriction (venturi) causes a low pressure inside the carburetor and high pressures outside the carburetor forces air and fuel inside

Fig 2 Air flows through the pilot screw and picks up fuel through the pilot jet. The pilot circuithas two adjustable parts, fig 2. The pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it 1

opens the passage moreand allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulated fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling and performance, the next smaller or bigger size pilot jet will be needed. The pilot jet is the part which supplies most of thefuel at low throttle openings. It has a small hole in it which restricts fuel flow though it. Both the pilot air screw and pilot jet affects carburetion from idle to around 1/4 throttle. The slide valve affects carburetion between 1/8 thru 1/2 throttle. It especially affects it between 1/8 and 1/4 and has a lesser affect up to 1/2. The slides come in various sizes and the size is determined byhow much is cutaway from the backside of it, fig 3. The larger the cutaway, the leaner the mixture (since more air is allowed through it) and the smaller the cutaway, the richer the mixture will be. Throttle valves have numbers on them that explains how much the cutaway is. If there is a 3 stamped into the slide, it has a 3.0mm cutaway, while a 1 will have a 1.0mm cutaway (which will be richer…