From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Service bulletin for the Stromberg AA-25 carburetor covering maximum power, acceleration, fuel level, and automatic choke control.
| Identifier | ExFiles\Box 122\2\ scan0252 | |
| Date | 1st January 1937 guessed | |
| PAGE 2 CADILLAC 37-60, 65, 70, 75, 1937 STROMBERG CARBURETOR SERVICE BULLETIN MAXIMUM POWER For maximum power or high speed running a richer mixture is required than that necessary for normal throttle opening. For part throttle opening, fuel is supplied through the main metering jet to approximately 75 M.P.H. At this position economizer valve (34) is forced down by the accelerating pump piston, allowing fuel to flow through economizer valve and discharging through pump discharge nozzle (37). Fuel is supplied continuously through these passages with throttle wide open. ACCELERATION For accelerating requirements it is necessary to supply momentarily an extra amount of fuel when the throttle is opened. On the up stroke of pump piston (31) fuel is drawn into the piston chamber through the inlet check valve (35). On the down stroke the compression closes the check valve and forces open economizer valve (34). The fuel is then discharged through pump discharge nozzle (37) into each of the carburetor barrels. When throttle is opened part way only a small amount of fuel is discharged. Two holes are provided in the throttle lever for accelerator pump rod. The rod is placed in the hole on the short radius for normal operating conditions. If a richer charge is necessary on acceleration, the rod can be placed in the hole on the long radius. Either position does not affect the opening of the economizer valve. [Diagram Key 1] 15. Float. 16. Float Lever. 17. Float Needle Valve Clip. 18. Float Fulcrum Pin. 19. Float Needle Valve. 20. Float Needle Valve Seat. 21. Float Hanger Gaskets. 22. Float Hanger. 23. Float Chamber Vent. 24. Pump Fulcrum Arm. 25. Pump Piston Link. 26. Felt Dust Washer. 27. Retainer Washer. 28. Dust Washer Spring. 29. Spring Retainer Washer. 30. Pump Duration Spring. 31. Pump Piston. 32. Pump Expansion Spring. 33. Pump Relief Valve. 34. Economizer By-Pass Valve. 35. Pump Inlet Check Valve. 36. Pump Discharge Channel. 37. Pump Discharge Nozzle. 38. Spark Control Hole. FUEL LEVEL Fuel level in the float chamber is maintained by floats (15). The level is set at the factory at 5/8" below the top surface of the float chamber which corresponds to the bottom of level sight plug (39), with the engine idling. The plug should be removed to observe the position of the level before the carburetor is dis-assembled to reset the float. It is not necessary to reset the float unless it has been tampered with or the carburetor has been handled roughly. If so, it can be reset by bending the float lever arm at the point adjacent to the fulcrum pin. When re-setting the position of the floats, Service Tool SER-292 should be used. This is accomplished by removing the air horn assembly and holding it in an inverted position. With the tool placed in position on the cover gasket, the top of the vertical guides of the tool should be flush with the tops of the floats when floats are properly set. The guides also serve to check the clearance between the floats. It is very essential that the floats be properly set so that they will not rub on the side of the main body and interfere with free movement. If the tool is not available the distance between the top of the float and the gasket should be 1-13/32". NOTE: If it is necessary to remove the air horn, remove choke valve screws so that valve, link and vacuum piston remain intact. IMPORTANT: Float level can be checked only while engine is running. DO NOT START ENGINE WHILE CARBURETOR COVER IS REMOVED BECAUSE A BACKFIRE FROM THE MANIFOLD MAY CAUSE A SERIOUS FIRE. [Diagram Key 2] 15. Float. 16. Float Lever. 24. Pump Fulcrum Arm. 39. Fuel Level Sight Plug. 40. Throttle Stop Screw. 41. Pump Rod. PAGE 3 CADILLAC 37-60, 65, 70, 75, 1937 STROMBERG CARBURETOR SERVICE BULLETIN GENERAL DESCRIPTION AND ADJUSTMENT OF AUTOMATIC CHOKE CONTROL In the Stromberg Model "AA{D. Abbot-Anderson}-25" carburetor the automatic choke control is built completely into the carburetor. The principle of operation is the same as that used on the previous Cadillac models. The tension of the thermostat is controlled by an electric heat unit connected directly to the choke valve stem making the entire unit intact. The vacuum piston used to open the choke valve, when the engine begins to fire, is also built into the body. In the Model "AA{D. Abbot-Anderson}-25" carburetor the fast idle feature which supplies fuel during cranking period and keeps the engine from stalling while warming up, is accomplished by a passage entering the throttle barrel below the throttle valve to feed the additional fuel with closed throttle. This design eliminates all external parts and adjustments used in the previous method of fast idle. Figs. 1-4 show the automatic choke control and fast idle in the various stages of operation. CHOKE OPEN—SLOW IDLE—ENGINE HOT Fig. 1 shows the choke valve "A" in wide open position, which it will be when a hot motor is stopped. In this illustration the carburetor is shown at slow or curb idle, which should be adjusted by the throttle stop screw at 6 M.P.H. with a hot engine and is the only adjustment required for the slow and fast idling of the engine. Vacuum piston "B" shown in the "lower" position, is connected to the choke valve by link "C". In addition to the piston opening the choke valve, it also controls the duration of the fast idle period. The thermostat is connected to the carburetor choke valve stem. CHOKE CLOSED—FAST IDLE As the engine becomes cold, the thermostat also cools and gradually gains tension to close choke valve "A". With this movement, vacuum piston "B" is raised, opening fast idle passage "E" so that air is drawn through "F" during the fast idle period. The suction of the air draws fuel from passage "F" to supply the additional fuel for the fast idle. During the cranking of the engine the choke valve remains in the position shown in Fig. 2. | ||