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).
Carburettor piston modifications, clearances, and main jet adjustments.
| Identifier | ExFiles\Box 1\10\ B001_X106 TO X 110-page079 | |
| Date | 24th October 1932 | |
| -2- HS{Lord Ernest Hives - Chair}/IHS.9/KT.24.10.32. this bringing the fully lifted position to 3500 R.P.M. full throttle, the heavier piston operating in a more satisfactory manner. By increasing the diametrical clearance of the lower piston ring from .004" to .006", keeping the top ring clearance at .004" the piston is further delayed in reaching the fully lifted position from 3500 to 4000 R.P.M. This we find is an unsatisfactory way of controlling the piston lift, as when the bottom ring clearance is increased the piston becomes less sensitive to the throttle position. From the results of our tests it appears that a diametrical clearance of .002 to .003" would avoid any risk of sticking and yet be sufficiently damped to overcome any violent initial jump when the throttle is snapped open. Assuming that the induction pipe depression is to be kept at its present figure, the weight of the piston and guide with the above clearance should be 15 ozs. in order that it reaches full lift at approximately 4000 R.P.M. We are having a piston of this combination for test. Trouble has been experienced in that the groove around the piston guide which retains the taper needle pin locking ring is not sufficiently deep to allow the ring to be below the surface of the guide when in position. Another feature is that the locking ring comes above and out of the guide bearing when the piston is in the fully lifted position. We have had brass inserts and cheese headed screws fitted to clamp down the suction chamber replacing studs as fitted, which necessitated dismantling the carburetter before the suction piston and chamber could be removed. Scheme to Lec.3243 which permits a .125 higher piston lift and slight increase in area past the large venturi has been tested and we find that the induction pipe depression and power at the high speeds are unaltered from our original figures. Main Jet and Taper Needle. The main jet we have opened up from an orifice .093 to .109 diameter, and the taper needle we are now using is .108 diameter at the large end and .065 diameter at the small end, with other dimensions as the original | ||