THE “CHOKE.”

King Country Chronicle, Volume XXX, Issue 4810, 18 February 1936, Page 3

THE “CHOKE.”

AN AID TO QUICK STARTING. Some owners of cars provided with “choking” devices which the driver operates from the dashboard make .the mistake of using them whenever * the engine is started, and also as a means of enriching the mixture should they consider that the cylinders are not getting enough “juice.” Such practices are unwise and have a cumulatively harmful effect on lubrication. The purpose of the “chock” is to provide a means of enriching the mixture temporarily when a cold engine has to be started. It should be unnecessary to employ it when the car is restarted while the engine is still warm. In a warning issued, stress is laid on the fact that while the 9 “choke” is in use the mixture fed to the cylinders contains an abnormal amount of petrol. That excess of fuel is not completely atomised, and some of it condenses on the walls of the induction manifold, while a proportion finds its way into the cylinders, and, if the engine is cold, is deposited on the cylinder walls.

The combustion of each over-rich charge is not complete, and the unburnt spirit works its way down the walls, "cutting" the film of lubricant which is their protection against wear and corrosion. Some of the excess fuel will probably percolate to the crankcase, where it causes serious contamination of the oil, for it needs but a very small percentage of motor spirit in the lubricant to produce marked dilution and reduction of its viscosity.

The only proper purpose of the "choke" is to enrich the mixture momentarily in order that certain difficulties associated with the starting of a cold engine can be overcome. When the starter-motor cranks the engine comparatively slowly it is hard if the engine is not warm to obtain a mixture of proper strength; that is to say, one which contains a right proportion of atomised fuel. To obtain efficient combustion a definite ratio of vapourised fuel to air is essential. Some motor spirits, being more volatile than others, tend to make starting easier, but in every case the warming-up of the induction manifold increases volatility. It is obvious therefore, that at very low engine speeds, such as prevail when the starter-motor is used, vaporisation is below normal, and the deficiency must be made good by employing the "choke," whose action is to restrict the intake of air and to create greater suction, which in turn produces a mixture richer in petrol. Although the "choke" is useful as an aid to quick starting, it should not be kept in operation after the engine has fired and will run steadily on moderate throttle. The best course is to let the engine idle fairly fast for a minute or two, since the greater velocity of the air stream will ensure that more petrol is picked up from the carburettor jets and, incidentally, the oil will be circulating freely by the time the engine has to take the load when the car is driven on the road. Frequently, when a cold engine is obstinate because of some small defect in the ignition, the

driver keeps on working the selfstarter and using full "choke." The first reluctance of the charges to fire might have been due to a poor spark and a rather lean mixture, but it is easy to go to the other extreme and fdl the cylinders with a top-heavy mixture which not even a brisk spark would ignite. If it is thought that the engine is stubborn for that reason, "choking" should be discontinued, and the cylinders cleared by switching off the ignition, opening the throttle fully, and spinning the engine for a few revolutions with the starter, so that the heavy mixture will be blown out through the exhaust valves.