]^72 BULLETIN 173, U. S. NATIONAL MUSEUM 



tained as the main and annular nozzles and the small orifice all 

 supply gasoline to the entering air. For slow running at light load 

 the throttle is nearly closed and the orifice alone supplies the gasoline 

 to the air. At normal speed and load the gravity chamber becomes 

 depleted and the carburetion of the air is mainly dependent upon 

 the inner main nozzle. 



These carburetors are still being made, though they have been to 

 a large extent supplanted by improved Zenith carburetors employing 

 the same compound-nozzle system but embodying automatic, built- 

 in, accelerating, economizing, and starting devices. 



ZENITH CARBURETOR, MODEL U, 1924 



U.S.N.M. no. 308343, original ; gift of tiie Zenith Carburetor Co. ; not illustrated. 

 This carburetor, similar to the one preceding, is sectioned and ex- 

 hibited with the Buda engine (U.S.N.M. no. 308340) described above. 



TILLOTSON CARBURETOR, MODEL SP-19C, 1926 



U.S.N.M. no. 300G13, original; gift of the Tillotson Manufacturing Co.; not 

 illustrated. 



This carburetor is of the type known as plain tube, as opposed to 

 air-valve and combination designs. In addition to a bypass or low- 

 speed jet and a main nozzle, the carburetor is equipped with an ac- 

 celerating pump and an economizer to meet the accelerating and 

 power requirements of 6- and 8-cylinder automobile engines. 



The economizer consists of a metering pin, or lift needle, in an 

 auxiliary fuel passage to the main nozzle. In use this pin is raised 

 by the choke control and allows a predetermined quantity of fuel to 

 deliver into the main nozzle to assist in warming a cold engine. The 

 pin is also connected to the fuel throttle so that it is raised at ap- 

 proximately full throttle to supply an additional flow of fuel for 

 maximum power requirements. 



Tlie accelerating pump is added for the purpose of temporarily 

 enriching the mixture for a sudden acceleration of the engine. It 

 consists of a piston operating in a fuel-filled well, which is in com- 

 munication with an accelerating jet. The piston is depressed when 

 the throttle lever is opened and a sudden opening of the throttle 

 forces the fuel in the well into the accelerating jet. Fuel that will 

 not immediately pass the restricted jet opening is driven into a grav- 

 ity chamber above the jet, from which it flows into the jet prolonging 

 the accelerating effect. 



