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).
Detailed description of carburettor adjustment mechanisms and their functions.
| Identifier | ExFiles\Box 122\1\ scan0032 | |
| Date | 3rd December 1928 | |
| OY3.R.12.3.28 - 3 - December 3rd, 1928. (3) Main Restriction. This is in the bottom of the float chamber regulated within narrow limits by an adjusting screw with notched head. The range of adjustment here would normally be only about ± 10% on fuel flow. (4) Economizer Needle. This is opened fully by a cam on the throttle spindle at full throttle and operates in parallel with the main restriction to supply a richer mixture at full throttle. In some way which I do not understand its effect is supposed to be nil under extreme high speed, so that it acts to fill in the "loop" of the part throttle curve and give a steady horizontal curve of say .08 richness throughout the range of full throttle running. The restriction for the economiser is changeable. (5) Accelerating Pump. This is also operated by the full throttle cam, and can be regulated in stroke by advancing or retarding the cam on the throttle spindle. The pump consists of a brass "bell" which is pushed downwards by the cam over a spring-held plunger. When the plunger descends it un-seals two small holes in the stud which guides the plunger, thus opening up the passage to the accelerating jet. The time which it takes for the spring plunger to return to its seat is the time during which extra fuel is discharged from the accelerating jet. In our case it discharges for about 3 seconds after opening the throttle. The accelerating jet is concentric with the main jet and passes up the center of the main jet. It has a small central hole which forms the changeable restriction for the accelerating pump, being set as small as possible to delay the pump action over as long a period as possible consistent with "instant response." These seven adjustments cover the running range of the carburetter namely:- Main Throat Main Jet Main Jet Air-bleeder Main Jet Needle Economizer Restriction Accelerating Pump Stroke Accelerating Pump Discharge Orifice (6) LOW SPEED ADJUSTMENT. This is arranged in the conventional position at the lower side of the throttle and is controlled by the angle at which the throttle valve is ground (i.e. 18º, 19º, 20º or 21º) by hand regulation of the air supply, and by the size of the air passages in the side of the main bore of the carburetter. -continued- | ||