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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).
Carburetor pilot jet modifications for improved cold starting and engine running.

Identifier  ExFiles\Box 74\2\  scan0052
Date  15th November 1921 guessed
  
OyS - G 161121
Sheet #3.

So far this change, carried out on half a dozen different manifolds, has refused to load up under any conditions.

We therefore believe that loading on the other pipes of this type has been due to the gas rushing up the drain pipes and so not allowing the fuel to drain down.

Pilot Jets:

The earlier type of pilot jet fixed on the carburetter was fitted to 4 cars and was successful at air temperatures above 50°F.{Mr Friese} Below this temperature the lubricating oil was too stiff and the device would not turn the engine over. It was still possible to get a start by opening the throttle slightly after the first few turns of the starter, and so using the pilot jet in a similar way to our present use of the priming device. (It is always necessary to leave the priming device on for a minute or so after a cold start in this country.)

The second type of pilot jet started and ran the engine well down to about the same temperatures after removing the spring and substituting a very soft spring of similar type to the throttle spring. (See attached print A-131F.)

We think this spring is stronger than need be since there is no hesitation about lifting the fuel to the jet, even with the engine turning over very slowly on the starter. We believe that the correct spring would only be just sufficient to hold the sliding choke in its "closed" position.

Below 50°F.{Mr Friese} the jet was not powerful enough to keep the engine running.

Opening up the jet by stages till it was .095" diameter gave us an excellent start at temperatures down to freezing. This, however, flooded raw fuel into the engine and caused piston knocks at starting due to breakdown of the oil film, when used with any other pipe than the vaporizer pipe.

The next step has been to increase the spraying action of the jet so as to use as little gasoline as possible.

The type of jet so far tried is shown in A-1371 enclosed. This gives a better spraying than the large circular jet, but still has to be set to pass a lot of fuel before the engines will start.

We have examined the spraying effect by inserting a glass tube between the pilot jet and the induction pipe.

We set it under a 7 ft. head to pass from 40 to 50 im-perial pints per hour. At settings much below this, it will not keep the engine running after a dead-cold start in freezing weather.
  
  


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