Small Engines Ag. 221

 Identify basic terms and definitions associated with carburetion  List and identify individual engine parts related to carburetion  Describe the operation of the three different types of carburetors

 Carburetion  To enrich fuel by mixing with air and transfer the mixture to the combustion chamber  Diaphragm  A thin, flexible disc that vibrates  Idle  To run disconnected so that power is not used for useful work

 Lean mixture  An air/fuel mixture with excess air  Rich mixture  An air/fuel mixture with excess fuel  Tachometer  A device used to indicate the speed of rotation (RPM)

 Vaporization  The act of mixing liquid fuel with air so that the liquid fuel is suspended by air to form an explosive mixture  Velocity  Quickness of motion

 To produce a mixture of fuel and air  Provide economical fuel consumption  Smooth engine operation over a wide range of speed

 Is a constant force that equalizes itself in a given area  Commonly between 13 and 15 psi  Air moves from high pressure to areas of low pressure  Carburetors create areas of low pressure utilize the force of atmospheric pressure  Venturi  Airfoil

 Used in a carburetor to form desired airflow patterns  Ex: a rock in a river  Upstream and around the sides of the rock, the velocity of the water is high  Downstream side of the rock, the velocity is reduced

 Artificially produces desired flow rates  Wind pressure is greater between two structures than it is in an open field  Rivers flow faster through narrow channels  The greater volume of air or water has to accelerate in order to maintain a constant volume flow

 Tank is above the carburetor, fuel flow by gravity (fuel tank cap is vented)  Fuel flows into the bowl  Float raises  Needle valve touches seat, shutting off fuel flow  Position of float at this time is called the “float level”  Float level  High enough to provide ample fuel at full throttle  Low enough to prevent flooding or leaking

 Needle valve  Also called main needle or high speed needle  Regulate the air/fuel mixture

 Throttle  A flat disc called a butterfly, mounted on a shaft, is placed in the carburetor throat above the venturi  Wide open throttle does not affect air flow  Closing throttle, decreases air flow, decreasing engine speed  At idle, throttle is almost all the way closed

 Idle mixture valve  Supplies fuel to keep engine running at an idle  Supplies fuel into the carburetor after the throttle

 Choke  Butterfly similar to a throttle plate, placed in the air horn  Air inlet flow can be closed or open to affect pressure in the venturi  Almost closed, creates a low pressure situation in the venturi, allowing more fuel (rich mixture) to aid in cold weather starting

 Fuel tank is below carburetor (is not gravity feed)  Vented cap on fuel tank keeps constant pressure in tank  Piston moving down in cylinder creates low pressure area in the carburetor throat  Difference in pressure forces fuel up the fuel pipe, past the needle valve and through two discharge holes

 Choke  Sliding plate mounted at outer end of carburetor  Pushed in for starting  Must be pulled out as soon as the engine starts, for proper engine function  Adjustments to Vacu-Jets should be made with the fuel tank half full

 Incorporates a diaphragm type fuel pump and a constant level fuel chamber

 Spring compression and expansion controls vacuum in the pump chamber  Pulsation of the diaphragm keeps a constant level of fuel in the fuel cup  One fuel pipe puts fuel into the fuel cup, the other takes fuel from the cup up into the pump chamber