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).
Testing and performance of different carburettor suction piston designs.
| Identifier | ExFiles\Box 13\3\ 03-page401 | |
| Date | 19th March 1932 | |
| -2- ES/Cwd.3/CA.19.3.32.contd The disadvantage to this scheme is the, slow fall of the piston when closing the throttle from full open at high speeds. If the throttle is again opened up before the piston has fallen, popping back occurs, as this is a similar condition to opening up a large fixed choke carburetter of the RR-C1 Type. We have tested a suction piston having a 45º seat with very good results. It has one .025 diameter suction hole in constant line of suction and four holes .062 diameter through its seat. When the throttle is snapped open, the piston is slowly lifted via the .025 hole and when off the seat is slowly carried on by the four .062 holes. This scheme gives a very quick fall of the suction piston a feature we desire. The reason for the slow opening is that the four large holes are not in the direct line of suction being drilled at right angles through the 45º seat of the piston. Prior to this scheme we had tested out a piston with masked suction holes to Lec.3007. If the holes are made to be just visible say .005 above the mask successful 'opening up' can be obtained. The disadvantages to this scheme are the minute adjustment of the position of the suction holes and the making of a needle valve to suit the masked portion and immediately the piston admits air to go on to the taper of the needle valve. Our 45º seat piston possesses desirable features. (a) Slow opening of the piston. (b) Rapid fall * * (c) The piston on a definite seat. The S.U. type of suction piston with steel guide which we are now using is ideal in its action. The bearing surface of the guide is of considerable length, can be kept nicely lubricated and is not exposed to any petrol, the result of a pop back of the carburetter. | ||