TM 5-4240-501-14&PSince no accelerator pump is used on this carburetorand since many of these engines are used on lawnmowers where rapid acceleration is needed, the mixtureshould be rich. Turn the needle valve in until the enginebegins to lose speed, indicating a lean mixture. Then,open the needle valve past the point of smooth operationuntil the engine just begins to run unevenly. Since thissetting is made without load, the mixture should operatethe engine satisfactorily under load.These carburetors do not have an idle valve, but themixture at idle speed is controlled in a different way. Asthe throttle closes to idle, the leading edge takes aposition between the two discharge holes. The larger ofthe discharge holes is now in the high pressure area, andthe flow of fuel through it will cease. The small hole willcontinue to discharge fuel but the amount will bemetered by the hole size and will be in proportion to thereduced air flow. For this reason it is important that thesmall discharge hole be of the proper size. The needlevalve will allow much more fuel to pass than should gothrough the small discharge hole. A number 68 drill canbe used as a plug gauge to check the small hole. Anumber 56 drill can be used to check the larger hole.This can be done with the needle valve and seatremoved. See Fig. 20.Figure 20.THEORIES OF OPERATIONCarburetionYou will notice a small section is milled out of the throttlewhere it meets the discharge hole. This concentrates theflow of air past the hole and assures good vaporization.The idle speed adjusting screw should be set to obtainan idle speed of 1750 RPM. This may seem fast topeople accustomed to auto engines, but it is necessaryin order to have fast acceleration. It also helps coolingand lubrication.A slight unevenness may be noticed at idle speed, butthis is normal and no readjustments of the needle valveshould be made.The choke is the sliding plate mounted at the outer endof the carburetor. Fig. 20 and 21. The choke is pushedin to close the air intake for starting but should be pulledout as soon as the engine starts. The use of this chokeshould be understood clearly. Many complaints ofengine trouble, upon investigation prove to be nothingmore than failure to properly use the choke, especiallywhere the choke is operated by a remote control. Thechoke must close fully.Figure 21.The latest engines with Vacu-Jet carburetors incorporatea ball check in the fuel pipe which assures a steady flowof fuel to the needle valve and discharge holes.1114
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