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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).
The initial chassis tests of a new semi-expanding carburettor, detailing performance, modifications, and starting issues.

Identifier  ExFiles\Box 13\3\  03-page419
Date  25th April 1932
  
To R.{Sir Henry Royce} From Hs{Lord Ernest Hives - Chair}/Lov.{Mr Lovesey}
c. to Sg.{Arthur F. Sidgreaves - MD} Wox.
c. to E.{Mr Elliott - Chief Engineer} Da.{Bernard Day - Chassis Design}
c. to By.{R.W. Bailey - Chief Engineer}

87080

Hs{Lord Ernest Hives - Chair}/Lov.{Mr Lovesey}5/MJ.25.4.32.

R.R. SEMI EXPANDING CARBURETTER.

We have now had the first tests on a chassis with the new semi-expanding carburetter and while sufficient time was not available to go thoroughly into all the points, due to the car going to France (19-EX), we were able to see that the results were promising.

The model fitted to 19-EX. incorporated all the modification found advisable from the test bed development and the air piston was the large diam. central guide type in which the suction lift is obtained through the holes in the diam. of the valve past the annular clearance between the piston and dashpot. The annular clearance in this case being .002 which permitted a steady lift under 'snap' acceleration and a reasonably rapid return when the throttle was shut.

A later piston having the base dia. the same as the dash-pot diam. - i.e. smaller dash-pot - has been instructed, but we did not have time to get this on 19-EX. before its departure for France.

The power output of this carburetter in its present form is less than with the large S.U. and we find that by removing the air valve completely at full power we increase the power from 155 to 162 B.H.P. or the same as the S.U. We intend to further analyse this power loss and consider we should look into the question of increasing the size of the extra air port.

We will briefly deal with the other points of this carburetter as fitted to 19-EX. :-

Starting.

Difficulty with restarting was experienced after the carburetter had been run dry after running out of fuel. A start up on the starter motor could not be effected unless the slow running petrol well was primed although only relatively few turns of the starter motor was sufficient to pump fuel into the float chamber.

The slow running supply was taken from just below the lower holes of the diffuser and it appeared for some reason that after the carburetter had been emptied the fuel would not easily find its way down through these holes to the slow running jet. This trouble was overcome by drilling from the base of the diffuser into the passage to the idling jet. It is probable that the upward displacement of the air in these passages was more readily accomplished than when the air had
  
  


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