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Adjustment of the Rolls-Royce 40/50 Silver Ghost Carburettor

Before any attempt to adjust a Silver Ghost carburettor it is vitally important that all other aspects of engine settings are in tune.  It is more than likely that any problems associated with poor performance or slow running are due to other problems not recognised.


Check List:


Valve Clearances: 

Gap 0.004” remove spark plugs then adjust each cylinder when cold with the piston at TDC on its firing stroke.  Remove floorboards and rotate engine via large pry-bar at flywheel starting from No1 and then each in turn in order of firing 1-4-2-6-3-5.  If you examine flywheel you will see TDC mark and in between fixing bolts at 60 degree intervals which you can use for aligning cylinders 4-2-3-5.


Cylinder Compressions: 

All should be within 10% of each other and at the very least 50lbs/sq/ins.


Spark Plugs:  

It is recommended to renew spark plugs.  Champion D16 or NGK AB6 or equivalent. Gaps: Battery ignition 0.025" Magneto ignition 0.020".  Positions: Magneto plugs over exhaust valves (or intermediate positions). Battery plugs over inlet valves.


Distributor Cap:  

Check segment clearances 0.004" (check for run out marks on inside of cap) clean away any debris.


Contact Points: 

Battery Ignition Distributor: Contact breaker point gap 0.020".

Magneto: Contact breaker points gap 0.015". 

Watford Magneto 0.015” to 0.020” dependant on type.

Distributor Trembler Coil: Contact breaker points gap 0.018” to 0.020”.


Note:  Contact Breaker Points and Magneto Points are Platinum plated and should not be cleaned to the detriment of the plating otherwise poor contact and performance will result. 


Battery & Magento Timing:

Check at BLI and MLI (Late) positions.

Air Leaks:

Check for air leaks in inlet manifold and all joints. Use a small diameter tube placed in one ear to probe around all joints.  Check for excessive wear in valve guides (a common place for air leaks).


Rolls-Royce Carburettor:




  1. Piston valve (throttle valve) free movement and no signs of wear.  

  2. Air valve should drop in 1-2 seconds and not fall more quickly.

  3. Air valve diaphragm to be within 0.001" of high-speed throat diameter; upper face to seat 0.025" above air ports.

  4. Air valve spring to have 1/32" clearance.

  5. Fuel level in jets to 3/32" to 3/16" below top.

  6. Float needle should hold petrol overnight without leaking with fuel tap in the on position but with tank pressure released.

  7. Low-speed valve should fit concentrically in jet and move freely.

  8. Set low-speed jet (valve) to lowest setting, as it settles into low-speed jet.

  9. Turn milled adjusting nut anti-clockwise over 195 degrees or 35 minutes. This is the mechanical setting, sufficient to start and run the engine at idling speed approx. 500rpm.

  10. Start engine and run till hot.

  11. Set governor so engine is running at just below the speed sufficient to start lifting air valve - this is slower than you would think.

  12. Adjust nut until two notches left of centre of STRONG/WEAK setting will cause engine to stall.

  13. Test finally by unscrewing air valve 1/2 to 1 turn - this should cause the engine to stop. If not, the diaphragm is in the ‘incorrect’ position. The ignition lever should be at retard (LATE) during the setting of the low-speed jet.

  14. Set high-speed by running engine at full speed, with the ignition fully advanced (EARLY). Clockwise turning of the high-speed adjusting nut will cause the engine to lean out, eventually failing with backfires and a loud sucking noise as the governor attempts to get more mixture by opening the throttle further. It is set correctly when the engine will begin failing when the mixture is set over towards WEAK, while developing full power at the centre position.

  15. Check the setting over the whole range from slow to fast by setting the governor up notch by notch from idle. At each speed the engine should begin to fail, or show signs of running lean, when the carburettor lever is set towards WEAK by the same amount. If not, shorten the air valve spring for a rich spot; lengthen it for a lean one.

  16. Test this finally on a road up a hill sufficient to need full throttle in top gear to maintain say 40mph, the engine should lack power with two notches WEAK and noticeably develop power with two notches STRONG. Your performance should now be in the range of 12-15 mpg or better. The most likely cause of failure, after all of the above have been attended to, is wear to the jets and valves.

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