from an absolute fracture of the porcelain, the main cause of stoppage is owing to the plug inside the cylinder becoming fouled with carbon through an imperfect mixture being used. Often too much lubricating oil is inserted in the crank-case. This then gets over, the top of the piston and is burnt up when an explosion takes place, leaving a heavy deposit of soot. When this occurs the plug should be removed and the points cleaned.
In long runs, when the motor becomes heated, the inlet valve-stem may stick on its seat through oil or the bye-products of the explosion getting on to the stems. A little petrol squirted by an ordinary bicycle oil-can on to the stem will generally overcome the difficulty. Another plan is to take out the inlet-valve and wash the stem with the finest black-lead and petrol. On evaporation the stem will be left well coated with black-lead, which is a very good lubricant where there is great heat.
In order that the motor should work well it is necessary to have good compression. In the four-cycle engine the charge is compressed every second stroke of the piston towards the head of the cylinder. To obtain good compression, which is the forcing of the mixture into a smaller area, there must be no leaks, and the cylinder, piston, and valves must be perfectly tight. Otherwise, when the mixture is reduced in volume it will leak out and there will be poor compression, with the result that the motor will not give anything like its proper power, the force of the explosion being greatly reduced. Such working, too, is not economical. It is necessary, therefore, to see that there are absolutely no leaks, and the points where leaking can occur are as follows:—(1) the inlet-valve; (2) at the sparking plug; (3) around the piston-rings; (4) at the exhaust-valve; (5) at the point between the explosion chamber and the cylinder-top, where the cylinder-head is fitted on to the cylinder. Every little leak, no matter how small, means a loss of power. The valves should be examined first, viz. the inlet and the exhaust. They should be packed with suitable washers, and it should be seen that they set firmly on their
