Consider how each of these indications may differ so that it is possible to locate the exact source of trouble without first investigating. If the car has been on the road for some time and the engine misses it will either miss regularly in one or more cylinders or irregularly in all cylinders. If the former, the miss is due to ignition. The cylinder in which the miss is occurring can be easily determined by short circuiting each plug with a screw driver. This is done by allowing the screw driver to touch the central electrode of the plug and also the engine. When a plug is short circuited and it does not affect the operation of the engine, it shows there was no spark jumping across the electrodes of the plug. If the cable to this plug is disconnected and held a short distance from the electrode of the plug from which it was removed, a spark will or will not jump this gap. If it does jump the gap it shows that the plug is short circuited. Then the plug is either carbonized or the insulator is broken. If a spark does not occur place the cable near the engine and if a spark occurs it shows that the gap was too large at the electrodes of the plug. If no spark occurs it shows that the trouble is not in the plug but at some point ahead of this. If the engine is firing on all but one cylinder the trouble must be some place between the distributor rotor and the plug. First see if the distributor is dirty and then check up the cable to see if it is broken or grounded. One point to be remembered is that the parts of the magneto or the battery ignition system incorporated in the instruments will affect the operation on all the cylinders and there is no need of looking for the trouble there if only one cylinder misses. If every other cylinder to fire misses and magneto ignition is used, it is often due to the time lever housing being jammed over to one side so that the interrupter points are opened only by one cam. In no case is it necessary to file the interrupter points to overcome a miss, for the interrupter affects the operation on every cylinder and not on one. If the miss is irregular it is due to carburetor or to fuel trouble. To locate the trouble open the pet cock at the bottom of the carburetor and if there is any water in the carburetor it will run out. This operation also shows whether or not the gas runs freely. If it does not there may be dirt in the line or no gasoline supply. After everything else has been tried to overcome the trouble, adjust the carburetor to compensate for too lean a mixture. When an engine is first started it will often miss. This is due to the engine being cold. Under no circumstances should time be wasted to overcome missing until the engine is warm. If an exhaust valve sticks it will cause the engine to miss as the gases will be forced out on the compression stroke. This is difficult to locate as it is a regular miss but usually results from an overheated engine.