TM 5-4240-501-14&PTHEORIES OF OPERATIONCarburetionAirfoilNow, what is an airfoil ? Here is a picture of a tube in anair stream. When still, the pressure is equal on all sides.Under movement, an air pattern is formed, Fig. 14, sothat we have a high pressure area and a very lowpressure area.Figure 14.Now how does all this apply to Briggs & Stratton enginesthat employe three types of carburetors, the Flo-jet(gravity feed or float type), the Vacu-jet (suction feed)and the newer Pulsa-jet (fuel pump) type?FLOW-JET CARBURETORS OR GRAVITY FEEDFirst, let us consider the gravity feed system. The tank isabove the carburetor and fuel flows by gravity. Notice anair vent hole in the tank cap so that air can flow in as fuelflows out and a vent hole in the carburetor bowl so thatair can flow out as fuel flows in. If one or both of theseholes were plugged, the flow of fuel would cease andstop the engine. See Fig. 15 and 16.Figure 15.Figure 16.As the fuel enters the bowl, it raises the float. The float inturn raises the needle in the float valve. When theneedle touches the seat, it shuts off the fuel flow, and theposition of the float at this time is called the float level.Float LevelThe float level in general should be high enough to affordan ample supply of fuel at full throttle and low enough toprevent flooding or leaking.To set the level on the carburetor, invert the upper bodyas shown. See Fig. 16. The float and the body covershould be parallel. If not, bend the tang on the float toobtain this position. The actual distance on the smallcarburetors is 5/16 of an inch between the float and thegasket. On the larger models it is 3/16 of an inch. It isseldom necessary to measure this distance. The floatlevel is not as critical as on some carburetors.Remember, however, that there should be one gasketbetween the float valve seat and the carburetor. Nogasket or two gaskets will change the float level.Now, the fuel is down into the bowl but how does it getinto the cylinder?Here is shown the position of the nozzle and the fuellevel. See Fig. 16. The fuel in the bowl seeks its ownlevel, which is well below the discharge holes. Noticethat the discharge holes are in the venturi, the place ofgreatest air velocity. As the piston in the cylindermoves down with the intake valve open, it creates a lowpressure area that extends down into the carburetorthroat and venturi. Two things start to happen.814
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