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 choke sizes and their effect on low-speed performance and driveability.
| Identifier | WestWitteringFiles\T\2January1929-June1929\ Scan115 | |
| Date | 7th February 1939 guessed | |
| contd :- -3- When the carburetter was fitted to a chassis it was found necessary to remove the 34 mm. choke, due to the poor pick up from slow speeds, and replace it with the 32 mm. choke. This smaller choke gave very good results and snap opening up could be obtained from speeds above 5 m.p.h. on top gear. This would be the speed and load at which the main diffuser started to operate. It is seldom that good opening up can be obtained with static carburetters from speeds below the change-over position when operating on a high gear. Consequently when large chokes are employed to permit of the development of the maximum torque the change-over position is raised to a higher speed which may make the car very unpleasant to drive on a high gear at low speeds - such as in traffic. Therefore for practical purposes on a touring car with a simple static carburetter it is usually found necessary to sacrifice a certain amount of possible power by keeping the chokes on the small side to obtain reasonable opening up at low speeds. From our test on the road report, it will be seen the considerable improvement in performance obtained and it must be remembered these tests were made with the 32 mm. choke. It can therefore be estimated the still further improvement we should expect if the larger - 34 mm. - choke were used if we refer to the BMEP. curve of the 34 mm. choke. Failure to give good opening up from low speeds is the only factor which prohibits the use of this larger choke. Some carburetters fit a pump or priming device operated by the throttle in an attempt to overcome this trouble contd:- | ||