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).
Carburetter performance, detailing issues with idling mixture and comparing different metering schemes.
| Identifier | ExFiles\Box 13\3\ 03-page316 | |
| Date | 15th May 1931 | |
| -2- Now closing the throttles down to the slowest idling speed it was found the adjusting screw opposite the club foot (No.2 on diagram) had to be fairly well opened. The quality of the mixture could not be affected much by adjustment of this screw. The quality could be altered by the outside adjusting screw. By screwing this right down the mixture was very much richened due to cutting off the air leak which takes place from below the throttle edge into the mixture passage and so diluting the mixture (and lessening the jet suction) issuing from opposite the "club foot". By the above means the mixture at the two extremes (i.e. "tick over" and just prior to the diffusers operating) could be fairly well controlled. Between these extremes the metering was not very good. On opening the throttle a little from the "tick over" the mixture became very weak. We can see that this is a question of the position of the discharge holes. Actually, the outer hole was acting most of the time as an air leak hole until the throttle had opened sufficiently for mixture to be drawn out above the throttle. We could still have further developed the scheme by varying the position of these discharge holes relative to the throttle edges and could probably have found the relative positions to satisfy the intermediate metering. It is not easy, however, to make these alterations and we should have needed another top half casting. Consequently, by a simple temporary modification we were quickly able to convert the system to that which we have found quite satisfactory on our aero carburetters. We cannot see that the two hole scheme on either edge of the throttle with its "club foot" possess any advantages. Undoubtedly it can be made to work alright as this scheme has always been a feature of the Solex carburetter. However, we have not found the Solex better in respect of the slow running metering than the one side metering schemes as used on Zenith, Claudel and Strombergs. Personally, we favour our standard aero effect i.e. variable position orifice. The scheme on the carburetter on 27-EX was only a makeshift but think that with design attention a very simple and neat eccentric adjustment could be produced like the latest scheme for the carburetters in the Vee. | ||