INSTRUCTIONS ON STROMBERG NA-S2 AND NA-S3 AIRCRAFT CARBURETORS Introduction The Stromberg NA-S2 and NA-S3 carburetors are designed to meet the exacting requirements of small two, three, and four eylinder aircraft en- gines, such as are used in small airplanes. The principles of operation as described in these in- structions are quite similar to those used in all Stromberg aireraft and motor car carburetors, The specification or setting in the carburetor is the result of a great deal of test work conducted by the engine and carburetor manufacturers in the laboratory and in flight, and should not be changed unless it is absolutely certain that a change is necessary to meet unusual operating conditions. Installation ‘The carburetor should be so mounted on the engine that the float chamber is at the side of the throttle barrel, preferably with the fuel inlet to the rear. With the carburetor in this position, the throttle control lever, which is adjustable to any radial position, is at the right side of the car- buretor as viewed from the rear of the engine. The fuel inlet is a 14” pipe tap connection located, at the back near the bottom of the main body if the carburetor is installed as above. When the fuel level is set at the factory a pressure of one~ half pound per square inch at the carburetor is used. As these carburetors will undoubtedly be used on engines having a gravity feed system, it is recommended that the tanks be located so that the minimum head of fuel on the carburetor inlet is twenty-four (24) inches under all normal con- ditions of flight, Starting As the carburetor is not equipped with a prim- ing device, the following procedure is recom- mended for starting. With the throttle closed the engine should be turned over two or three times before the ignition is turned on. This will draw fuel up through the idle system and then if the ignition is turned on the engine will usually start fon the next turn over. AS soon as the engine starts to fire, it is usually necessary to open the throttle slightly to keep the engine running and to warm it up sufficiently for norma} operation. Adjustment ‘The main metering jet used in the carburetor is of the fixed orifice type, and its size as well as the remainder of the carburetor specifications has been determined by test work as previously men- tioned, so that no adjustment for cruising and full orm No, 10:148:3000-10-398PI_ Printed in United Stat of Ameren + throttle speeds is required. An idle adjustment is provided to take care of slight production varia~ tions in the carburetors, and engines. A small Knurled screw near the edge of the butterfly valve, on the throttle valve body, may be adjust ed to control the richnessof the mixture at idling speeds, Turning this screw in a clockwise direc- tion closes off the passage leading to the upper idle discharge hole and leans out the idle mixture. ‘Turning in the opposite direction of course gives a richer mixture. CAUTION! Ii the Idle Ad- justing Needle is screwed into its seat with too much force the needle will be grooved and the needle seat in the throttle body will be damaged. This will prevent an accurate idle adjustment and will generally necessitate replacement of the needle and throttle body. Great care should therefore be exercised when adjustment is being made not to turn the needle into its seat more than finger tight. In case the needie has been serewed into its seat by the fingers and the en- gine is still apparently idling too rich, it is sug- gested that the engine primer be inspected to See if it is allowing uel to enter the engine. But under no circumstances should the Idle Needle be screwed in with a pair of pliers or by using much force on a screw driver. A throttle stop is pro- vided on the throttle shait next to the throttle control lever, which should be adjusted to obtain the desired idling speed. Both the throttle stop and the idle adjustment should be set with the engine hot to obtain the proper idling speed and smooth operation. Servicing Once the carburetor is properly installed and the idle adjustment made, very little attention is required in service. A fuel strainer is provided near the fuel inlet of the carburetor, and may be removed by the removal of the large hexagon head plug on the side of the float chamber. A small square head plug is provided as a drain in the bottom of the carburetor. The strainer and drain plug should be removed frequently to get rid of any dirt or water which may have accumu- lated in the strainer chamber of the float cham- ber. The entire carburetor should also be in- spected to see that all parts are tight and prop- erly saietied. Description and Functioning of Carburetor FLOAT MECHANISM: A conventional hinge type of float mechanism located in a float cham ber having ample fuel capacity to operate in all ordinary maneuvers is used. This float mechan- ism is adjusted at the factory to obtain the proper Page Onefuel level, and requires no adjuistment in service unless it is necessary after a long period of serv- ice to install new parts, For information con- cerning the proper level see the section of these instructions pertaining to “Overhaul.” MAIN METERING SYSTEM: The metering. system used in the carburetor is of the plain tube type with an air bleed to the main discharge nozzle. The main discharge nozzle is located at the center of the venturi and is screwed into a boss projecting into the air intake. The main air bleeder is screwed into the air bleed arm which is held in place by the main discharge nozzle. The actual metering of the fuel is accomplished by the main metering jet which is assembled in the bot- tom of the float chamber in a channel through which the gas flows to the main discharge nozzle. ‘The size of the main metering jet affects the fuel consumption at all speeds from approximately 1000 RPM to full throttle speed. x IDLING SYSTEM; Inasmuch as the main me- tering system will not function at very low air flows (low engine speed), an idling system is pro- vided. This consists of an idle tube with an idle Inetering orifice in the bottom and several ait bleed hotes in the wall, an idle air bleed, and two holes in the throttle barrel, which act as idle dis- charge nozzles. A needle valve type of adjust- ment is provided on the upper discharge nozzle, Which regulates the quality of the idle mixture Fuel for the idle system is taken from the annular space around the main discharge nozzle, passes through the idle metering jet and mixes with the air from the idle air bleed located in the main body behind the venturi. ‘The air enters the tube through the bleed holes and the mixture then passes out of the upper or lower idle discharge hole. ‘The relative quantities passing through the tipper and lower idle hole depends upon the posi- tion of the throttle. At extreme idle, all the fuel passes through the upper hole and as the throttle Opening is increased, more and more of it passes through the lower hole, The idle system operates up to-an engine speed of approximately 900 to 1600 RPM. Overhaul DISASSEMBLY: The carburetor should be disassembled for cleaning and inspection each time the engine is given an overhaul. After the carburetor has been removed from the engine and the hot spot and air intake or heater taken off, the halves of the carburetors may be sepax rated by the removal of the filister head screws at the parting surface. ‘The venturi fs held in the lower half by 2 hexagon head serew. Remove the set screw which holds the float fulerum pin in place and the plug at the side of the carburetor, which will permit the removal of the float fulerum pin. The float and the float needle valve will then come out and it will be possible to remove the main metering jet, which Page Two is located below the float, Remove the idle tube which is screwed into the main body. If there is any indication of dirt or foreign matter in the float chamber, it is advisable to remove the main discharge nozzle. The removal of the above parts will permit a thorough inspection and cleaning of the carburetor, and unless replacements are ne- cessary, further disassembly is not recommended, INSPECTION AND CLEANING: The bodies and all parts should be thoroughly cleaned in gas- line, and all passages blown out with an air hose, ‘The float needle valve and seat should be in~ spected for wear and if the needle valve is badly grooved, both parts should be replaced. The needle valve is made of stainless steel and the seat of naval brass so that under ordinary service con- ditions these parts should last for many hundreds of hours. Check the main metering jet and float needle seat to make sure that they are tight. It is important that the throttle valve fits the barrel tightly when in the closed position and that the lower edge be flush with the top of the lower idle REPLACEMENTS: If due to accident or wear after long service it is necessary to make replace ments, the parts should be obtained from the Bendix Stromberg Carburetor Company, South Bend, Indiana, or an authorized Stromberg air- craft carburetor service organization, In ordering parts, be sure to state on what make and model engine the carburetor is being used as size of some parts are different for differ ent engines. Ordering by part numbers as shown on the attached assembly drawing and also giving the serial number of the carburetor will greatly facilitate service. REASSEMBLY: All headless screw plugs be- low the fuel level should be assembled with shel- lac, being careful not to get it on the end of the plug where it will come off and be carried by the fuel into one of the metering orifices. Headless serew plugs above the fuel level and all other threaded parts screwed into the bodies should have a compound of graphite and castor oil put on the threads. ‘The float level on these carburetors should be 13/37" below the parting surface and is dependent upon the thickness of the gasket under the needle valve seat, ‘The level should be checked under the same conditions encountered in service as regards the fuel used and the fuel pressure or head at the carburetor. The levels are set at the factory with a presse at the carburetor of one-half pound per sq. inch (19° gasoline at 710), and this is Fecommended for setting the levels in the field Te after fitting new parts, the level is not correct, remove the needle valve’seat and put in thicker gaskets to lower the level, and thinner gaskets to aise it. Onevsixty-fourth inch change'in gasket thickness will change the level approximately 5/64". ; |(03376 uly 3301 [390H NoILONS NOD “~LeIN—~ suriz~a z= fesceseh ueesi-a} 9pzz-ael\, leeesi- + "> 4 3 i” 7 q 9) ees @ (uamor) 30H HSI FII—~ ee v (waaan) 30H HOsia 310\— 18019-40509 volesfi Guvanos BoLsuTEIV) SUaEHOMIS MONTEBENDIX STROMBERG CARBURETOR COMPANY