Timers often make good contact when starting, or at low speed, and misfire badly at high speed. This will be caused generally by the contact sectors or insulation projecting beyond one another, the roller has time to make good contact at low speed but jumps over the sector at high. The roller I may become rough or develop a flat stop which will cause it to jump over the contact occasionally, or it may become loose on its bearing pin J, causing intermittent misfiring. The wearing or loosening of pins J and X result in poor contact. Should pin J fall out of the lever H, the roller would drop out of the fork and cause serious damage. This has happened in two cases to my knowledge. Should the spring G weaken or break, contact will be made intermittently at high speed, and no contact at low. In this case it would probably be impossible to start the engine. In case the spring breaks, a rubber band may be used temporarily. Wire connections to the timer should be examined frequently as the continual back and forth movement tends to twist and loosen the wire. Use stranded or flexible wire for these connections, if possible. Before removing the timer mark the hub and the shaft so that the hub can be properly replaced. If this is not done the engine will be out of time with the usual results of hammering or power loss. Should the gears which drive timer shaft be removed, be sure and mark the teeth of both gears in such a manner that there will be no mistake possible in reassembling them. Mark a tooth on the small gear by scratching or with a center punch (the tooth selected should be in mesh with the large gear). Then mark the two teeth of the large gear that lay on either side of the marked tooth of the small gear. Thus it will be easy to locate the proper relative position of the two gears at any time. (95) High Tension Spark Plug. The high tension spark plug is a device that introduces the spark gap and spark into the combustion chamber, and at the same time insulates the current carrying conductor from the cylinder walls. Since the voltage of the jump spark current is very high it is evident that the insulation of the plugs must be of a very high order and that this insulation must be capable of withstanding the high temperature of the combustion chamber. A cross-section of a typical plug is shown by Fig. 98, together with its connections and the course of the current, the latter being shown by the arrow heads. The electrode B through which the current enters the cylinder is thoroughly insulated from the walls by the porcelain rod C. The porcelain forms a gas tight joint with the threaded metal bushing F at the point P, the tension caused by the electrode B and the nut I holds the porcelain firmly on its seat at P.