fu^i systems ^he engine operates on a mixture of air and fuel. Tne fue| system has two main parts, as shown in Figure 9_i. The fuel tank or gas tank allows the engine to store enough fuel for several hours of °Peration. Tne engine part used to mix air and fUei together is called a carburetor. There are rn.any types of carburetors. They all work in the Same basic way. In this unit we will study the °Peration of the fuel system. LET'S FIND OUT: When you finish reading and stU(ly. this unit, you should be able to: 1. Explain the parts and operatj0n of a baSic carburetor. 2. Describe the parts and operation of a vacuum or suction carburetor. 3. Describe the parts and operation of a float carburetor. 4. Describe the parts and operatj()n of a diaphragm carburetor. 5. Explain the operation of a s|jdi yalve carburetor. 90 SMALL ENGINE SYSTEMS THE CARBURETOR Venturi One of the engine's strokes is called the intake The middle of our carburetor tube has a par• stroke. On this stroke the piston moves down very tially restricted area. This area is called the ven• fast. At the same time, the intake valve is opened. turi, Figure 9-3. Air has a hard time getting This fast downward movement causes a vacuum through the venturi. As the air does get through, it in the cylinder, and this is how the air-fuel mix• speeds up, leaving low pressure in the venturi. ture gets into the cylinder. This low pressure will pull in fuel. The carburetor's job is to mix the air and fuel together in just the right amount. Next to the Fuel Pickup intake valve, the carburetor is attached to the engine. A carburetor is shaped like a tube, as We must have a way of mixing fuel with the air. shown in Figure 9-2. The tube is open at one end. A small amount of fuel is stored in the carburetor, When the piston is on the intake stroke, air is as shown in Figure 9-4. The fuel is right under the pulled in the open end of the carburetor. The air venturi. A hollow pickup tube runs from the fuel can get into the engine through the open intake to the venturi. valve. When air goes through the venturi. it causes a low pressure. This low pressure sucks fuel up the pickup tube. The fuel mixes with the air. The mixture of air and fuel goes into the engine. AIR FUEL DOWN ON INTAKE r~| STROKE <Jr YJl 1 Figure 9-4. The pickup tube lets fuel mix with the air. Throttle Valve Figure 9-2. A carburetor is a tube open at one end, attached to the engine. There are times when we want an engine to run AIR fast. At other times we want it to run slowly. If a large amount of air-fuel mixture enters, the engine runs fast. If just a little air-fuel mixture gets in, the engine will run slowly. We use a part called the throttle valve to con• trol engine speed. A throttle valve is shown in Figure 9-5. The throttle valve is a round plate. It fits in the end of the carburetor tube. The valve Figure 9-3. The venturi is a raised part of the carbure• may be opened or closed by the person using the tor that causes a low-pressure area. engine. FUEL SYSTEMS 91 Choke When an engine is cold, it is hard to start. A carburetor has a choke, Figure 9-8, which helps start a cold engine. The choke is a small round valve. It looks a lot like the throttle valve. The choke valve fits in the carburetor near where the air comes in. When the engine is cold, we close the choke valve. Very little air can get into the carburetor. The piston going down causes a vacuum. This Figure 9-5. The throttle valve fits in the end of the strong vacuum pulls a lot of fuel out of the pick• carburetor. up tube. The fuel goes into the cylinder. All this fuel helps the engine start more easily. The choke valve usually is connected to a lever, Figure 9-9. The lever is on the outside of the carburetor. We use the level to close the choke valve when the engine is cold. We move the lever VALVE CLOSED to open the choke valve when the engine is warm. Figure 9-6. A closed throttle valve prevents air and AIR fuel from getting into the engine. CHOKE VALVE T CLOSED FUEL AIR OPEN ,i THROTTLE Figure 9-8. When the choke valve is closed, a lot of fuel is pulled up the pickup tube. Figure 9-7. An open throttle valve lets in a large amount of air-fuel mixture. A closed valve is shown in Figure 9-6. Very little air and fuel can get into the engine, so the engine runs slowly. The more the valve is opened, the more air-fuel mixture gets into the engine, and the faster the engine runs. An open throttle valve Figure 9-9. A choke lever is used to open and close the is shown in Figure 9-7. choke valve. 92 SMALL ENGINE SYSTEMS High-Speed Fuel Adjustment Screw THROTTLE VALVE When the engine runs at high speed, it needs a lot of air and fuel. Air enters the carburetor and goes through the venturi. A vacuum in the venturi pulls fuel up the pickup tube. The fuel mixes with the air and enters the cylinder. We need a way of controlling the amount of fuel that goes up the pickup tube. A screw with a pointed end fits in the side of the carburetor as shown in Figure 9-10. The end of the screw goes LOW SPEED ADJUSTMENT into the pickup tube. We use the screw to open or SCREW close the pickup tube. If we turn the screw all the Figure 9-11. The low-speed screw controls the fuel way in, not much fuel can go up the tube. If we coming out behind the throttle valve. turn the screw out, a lot of fuel can go up the tube. This screw is called the high-speed fuel adjust• ment screw. We use it to adjust the amount of fuel XX going into the engine at high speed. In a later unit we will see how to adjust this screw. THROTTLE I FUEL VALVE OPEN Figure 9-12. Low-speed adjustment screw. C imiiiwmiiiiS' HIGH SPEED ADJUSTING SCREW ED Figure 9-10. The high-speed adjustment screw con• trols the fuel coming up the pickup tube. Low-Speed Fuel Adjustment Screw HIGH SPEED When we want an engine to run slowly, we close the throttle valve, so that very little air goes Figure 9-13. The low-speed and high-speed adjust• through the venturi and very little fuel is pulled up ment screws and the choke valve can be seen in this the pickup tube. We need a way to get a small carburetor. amount of fuel into the engine so it will run. We do this by having a small hole behind the throttle valve. Fuel is pulled through a passage and out pointed end is used, Figure 9-12. The screw goes the hole as shown in Figure 9-11. through the side of the carburetor. The pointed We need a way to control the amount of fuel end goes into the fuel passage. If the screw is that comes out the hole. A small screw with a turned all the way in, very little fuel can come out FUEL SYSTEMS 93 the hole. If the screw is turned out, more fuel can get out the hole. We will learn how to adjust this / screw in a later unit. The carburetor shown in Figure 9-13 has the low-speed fuel adjustment THROTTLE f screw, the high-speed fuel adjustment screw and the choke valve identified. SUCTION CARBURETOR The suction or vacuum carburetor is one of the most common carburetors. Many small engines use a vacuum carburetor. A vacuum carburetor is HOLES FOR MOUNTING mounted on top of a fuel tank. Fuel is pulled out of the fuel tank by a vacuum. All vacuum carbure• Figure 9-15. Parts of a vacuum carburetor. (Briggs & tors work the same. They are sometimes called Stratton Corp.) vacu-feed or vacu-jet carburetors. A throttle valve is inside the carburetor. Right Suction Carburetor Parts below the throttle valve is a tube called the fuel pipe, which brings fuel up into the carburetor. A A vacuum carburetor is shown in Figure 9-14. fuel pipe is shown in Figure'9-15. There is a small The carburetor has two bolt holes used to mount screen in the bottom of the pipe. The screen stops it to the engine. There is an opening at the top of dirt from going up the pipe and into the carbure• the carburetor, which allows air to enter. The air tor. A small ball fits in the bottom of the pipe, goes through the center of the carburetor. A Figure 9-16. The ball lets fuel go up the pipe, but it choke is mounted next to the air opening. The will not let fuel run back out of the pipe. If the choke can be used to open or close the air pipe is empty, it takes a long time to start the opening.