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).
Analysis of carburettor functions including starting, breather, idling, and sizing for various engines.
| Identifier | ExFiles\Box 122\1\ scan0033 | |
| Date | 3rd December 1928 | |
| OY3.R.12.3.28 - 4 - December 3rd, 1928. Sketch SK-263 shows how air flow in the lower of the two air passages is reversed as the throttle opens increasing the suction on the jet and preventing a lean spot in operation. As the ground-angle of the throttle is altered the throttle opening at which this reversal takes place can be changed. (7) Starting. The choke valve is intended to close entirely and is ground at an angle like a throttle valve. This seems too drastic treatment but the makers say it is necessary for starting in really severe weather. In addition to the choke an auxiliary needle valve is used which is lifted an adjustable amount when the choke is closed. This supplies fuel direct to the wall of the main throat. (8) Breather. The hole on the carburetter side of the choke valve is not for a crankcase breather but for an electric "fumer" for starting of small racing engines in cold weather. We have used it for the crankcase breather by covering it with a plate having a 1/8" restriction hole. We have no trouble starting. In A-12900 we show the breather taken to the outer side of the choke valve. L-286 attached shows the installation of the standard Stromberg carburetter of the type which is being sent to you. The above covers the main functions of the carburetter. There is of course a clamped-screw adjustment for the idle position, which we find excellent for obtaining very slow and silent idling. An incidental advantage of using a carburetter of this type is that it can easily be arranged for the tappet covers to be removable without dismantling any other parts of the car. Size. The 1-3/4" size is recommended by the makers for the Phantom having a displacement of 103 cu. ins. per foot. It is actually used on the Chrysler 80 having a displacement of 80.5 cu. ins. per foot with a 1-7/16" throat. This makes me think that we might do better with a larger throat. It is harder to get smooth acceleration without lean spots as the throat is increased. On the 20 HP. they would recommend a 1" carburetter. As an example the Chrysler 65 has a 1" Stromberg UX carburetter. -continued- | ||