Today I have been carrying out a few rather rewarding odd jobs on a MK6 Bentley. The owner and I rebuilt the engine some years ago and since then we have never got around to making it run quite as well as it should. When it first went back on the road we had to confront some rather unfortunate carburettor problems due to an unusual sequence of component faults: One of the new floats let in petrol and after we had replaced it with a good secondhand item the engine was still running so rich that it fouled spark plugs. Further investigation revealed that the new carburettor jets were oversized and that the needles were in places several ‘thou’ smaller than they should have been. At the time I machined out some worn .090″ jets to .100″ and we re-fitted the original needles which made the engine run acceptably well.
More recently we have had the time to fit new jets and needles which do conform to specification and the car has started to run really rather well although cold starting has been poor and the combustion witness on the plugs looked rather rich. Today we fitted a new set of plugs which allowed the mixture to be weakened by a flat or so on the adjustment nuts and all of a sudden things are working as they should: The engine is completely smooth with an even exhaust beat, power is excellent and the combustion witness on the plugs is just right.
I have often found that when spark plugs have had a hard life the best thing is to replace them and this seems particularly to be the case now we are using unleaded fuel. It’s also worth noting that spark plugs are made in Japan. Also available are various ignition devices with a gap between the electrodes and which come from British manufacturers; I have a few in stock and on the odd occasion on which I supply them, they come free of charge and without warranty…..
Having sorted out the combustion issues with the MK6 the last item on the list was an engine oil change in case unburned fuel had previously found its way into the sump. This particular example was built in 1946 and like all cars of its day would have been expected to use a little oil. These days it uses no measurable quantity and this is due to the advances which have been made in the design of pistons and rings. It is interesting to note that at one of the first meetings of the Society of Motor Manufacturers and Traders after the Second World War, Capt. G.T. Smith Clarke delivered a paper which called for a change in the practise of producing big end bearing assemblies with narrow side clearance. This practise was intended to limit the amount of oil thrown from the crankshaft journals onto the cylinder bores in order to reduce oil consumption. Smith Clarke instead proposed wide big end side clearance and the development of better oil control rings for the pistons. This, he believed, would better lubricate the cylinder bores and extend engine life. History has proved him to be corrrect and the better manufacturers of pistons for older vehicles have appropriately used modern design to give lower oil consumption and longer piston life.