TM 5-4240-501-14&PTHEORIES OF OPERATIONCarburetionThe idle valve chamber leads into the carburetor throatabove the throttle. Here the pressure is low, and the fuelrises in the nozzle past the idle valve and into thecarburetor throat through the discharge slot. Theamount of fuel is metered by turning the idle valve in orout until the proper mixture is obtained. Here again wesee what happens if the needle is screwed in too far. Adamaged idle valve can result.Adjustment of the idle valve is similar to that of theneedle valve but should be made after the needle valvehas been adjusted. The idle speed is not the slowestspeed at which the engine will run. On small engines it is1750 RPM. On larger engines the idle speed may be aslow as 1200 RPM. Use a tachometer to set the speed.Turn the idle speed adjusting screw (located on throttleshaft) until the desired idle speed is obtained and holdthrottle closed. Turn the idle valve in until speeddecreases, then out until speed increases and againdecreases. Then turn the idle valve to a point midwaybetween these two settings. Usually the idle speedadjusting screw will have to be reset to the desired idlespeed.Figure 18.The next problem is starting the engine in differenttemperatures and with different fuels. A butterfly,mounted on a shaft, is placed in the air horn. With thischoke we can close, or almost close, the air horn and geta low pressure area in the venturi and throat. See Fig.18.Thus, a rush of fuel is obtained from the nozzle with arelatively small amount of air. Even with low vaporizationthis extra rich mixture will give easy starting. Only aportion of the fuel will be consumed while choking, and alarge portion will remain in the cylinder. This rawgasoline will dilute the crankcase oil and may even causescuffing due to washing away of the oil film from betweenthe piston rings and the cylinder wall. For this reason,prolonged choking should be avoided.This now is our complete carburetor.VACU-JET CARBURETORS ORSUCTION FEEDNow let us take a look at the Vacu-jet or suction feedsystem. Here the fuel tank is below the carburetor, soobviously the fuel will not flow by means of gravity.Therefore, the force of atmospheric pressure must beemployed.Figure 19.Again we have a vent hole in the fuel tank cap to allowthe pressure in the tank to remain constant. Now here issomething important. Before adjusting the carburetorpour in enough fuel to HALF fill the tank. The distancethe fuel has to be lifted will affect the adjustment. At halffull we have an average operating condition, and theadjustment will be satisfactory if the engine is run withthe tank full or nearly empty.As the piston goes down in the cylinder with both theintake valve and the throttle open, a low pressure area iscreated in the carburetor throat. A slight restriction isplaced between the air horn and the carburetor throat atthe choke. This helps to maintain the low pressure.The difference in pressure between the tank and thecarburetor throat forces the fuel up the fuel pipe, past theneedle valve, through the two discharge holes. Thethrottle is relatively thick, so we have, in effect, a venturiat this point, thus aiding vaporization. A spiral is placedin the throat to help acceleration and also to help keepthe engine from dying when the throttle is openedsuddenly.The amount of fuel at operating speed is metered by theneedle valve and seat. Turning the needle valve in orout changes the setting until the proper mixture isobtained. This adjustment must always be done whilethe engine is running at operating speed, not at idlespeed. While the needle valve may look like an idlevalve due to its position, it is a true high speed mixtureadjusting valve.104
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