Habe das in meinem Mk6 Register Newsletter gefunden. Woran erkenne ich, ob es eine SP oder SN Nadel ist?
A note on Bentley post WWII 4.5 litre engine SU carburettor needles ---
This engine was fitted to the MK VI from the M series and all R types. Equivalent Rolls Royces with the 4.5 litre engine were fitted with a single downdraught carburettor to limit the performance.
The Service Handbook states that the correct needle for the SUs on the 4.5 litre Bentley cars is the SP type. However, if one reads the individual Service Bulletins concerning the Cooling System it explains that early 4.5 litre engines were fitted with SN needles in the SU carburettors. Following complaints of overheating from customers (NB even when the cars were fairly new!) a change was made to a richer SP needle, which would both cool the engine and increase the engine power.
The author‟s Bentley was fitted with the SN needles and these were changed to SP during the restoration project, following the Service Handbook information. Economy was 16.5 mpg at 70ish, 18mpg at 60 & 20mpg tootling along at 50/55. After discovering the above, and considering that the modern core in the author‟s radiator gives a much better cooling than the original, the SN needles were replaced to give 19.5mpg at 70. What joy! And the performance is still very adequate with the standard, peak revs at 100mphish gearing.
The above was achieved with the mixture set as weak as possible without popping the exhaust on the overrun. This was equivalent to 2.5% CO with SP needles and 0.0 % CO with the SN needles. One presumes that the valves are made of RR quality material! Note that in order to follow the Crewe original practise and have an even, low speed idle, it is necessary to set the mixture to about 5% CO. This is equivalent to setting the mixture lever at the letter “E” of “MIXTURE” on the steering boss with the above setup.
