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).
Modifications and road trials of an S.U. carburetter for a 20/25 HP unit.
Identifier | ExFiles\Box 31\5\ Scan115 | |
Date | 1st October 1933 | |
1 5/8 S. U. CARBURETTER TO HB.{C. E. Harcombe}1157. A carburetter modified to the above scheme incorporating Hs.{Lord Ernest Hives - Chair}1139 has been tried on the road running on a standard 20/25 HP. unit. From previous tests it has been proved that while quite good slow running can be obtained on the standard S.U. carburetter, the tick-over is inconsistent and therefore unreliable. One reason for this may be attributed to the fact that the piston is lifted when the engine is only slowly ticking over, and there is sufficient friction present to prevent it taking up exactly the same position every time on slowing down, when slow running is required. With the throttle edge type of slow running device fitted the piston has an initial lift of .250", and so running below 750 R.P.M. the piston is resting on a stop. For starting from cold, the throttle edge slow runner is not effective enough to act alone as a starting medium, and it has been found necessary to fit an automatic stop (Hs.{Lord Ernest Hives - Chair}1139), which in working in conjunction with the mixture control, allows the piston to drop to the same position as on a standard S.U. carb. (in the full strong position). This has the advantage of bringing into action the main jet when the throttle is opened, and the engine turned over A further point to consider, in coupling the piston stop with the mixture control, is variation of mixture strength while the engine is still being warmed up. The higher the initial lift of the piston, the later the main jet will come into action, due to decreased air velocity. When the engine is cold, the mixture from the throttle edge is not strong enough to keep the engine running, so by adjustment of the mixture control (which is necessary in any carburetter) the coming in of the main jet is adjusted as required. For preliminary tests, the throttle edge slow runner was put on the underside of the throttle body, but in further experiments it will be tried on the upper side of the throttle body, as something may be gained by eliminating the risk of raw petrol lying about in the bottom of the pipe, interfering with the action of the slow running jet. Hs{Lord Ernest Hives - Chair}/C. T. Salt. | ||