If the engine stops on the road and pressing the starter pedal fails to start it, or if one or two cylinders miss fire, the first thing to do is to get the crank out of the tool kit and crank over the engine. If, with the gears in neutral, the engine cranks over hard, it indicates a lack of lubricating oil, or a lack of water, which has allowed the engine to reach a temperature where the lubricant fails to perform its work. If the engine turns over fairly easy, it is not necessary to look for oil or water trouble.
The next test is for compression. If the driver is not experienced and is unable to tell simply by the resistance to the starting crank whether each cylinder has compression, he should open all petcocks except on one cylinder and turn the crank two revolutions, noting if there is a resistance for one-quarter of a revolution in the two complete turns. Compression occurs only on one stroke of the piston in the four-stroke cycle. Each cylinder should be tested in a similar manner, opening all petcocks except on the cylinder being tested; see if the compression is practically equal in all cylinders.
If one cylinder has very weak or no compression, the trouble will be found usually in the exhaust valve. First examine the push rod to see if there is clearance between it and the valve when the valve is supposed to be closed; if there is, the valve must be lifted out and the valve and seat inspected for carbon. Sometimes a piece of carbon will lodge on the valve seat and, due to the hammering of the valve, will become fastened to valve or seat. For temporary repair generally it can be scraped off with a knife, and the valve be ground in upon reaching the garage.
If the trouble is not in the exhaust valve, it might be in the inlet valve. In some types of engines the valve head may break off and get into the cylinder and when the piston comes up punch a hole in the piston head. A petcock may be loose so that it will jar open sufficiently to affect the compression and so cause the cylinder to miss fire. These troubles usually are confined to one cylinder and not to the whole engine.
The gasoline is the next to be inspected. Is there any gasoline in the bowl of the carburetor? This may be determined by inspection, opening the drain cock, or “tickling”—flooding. If not, examine the gasoline tank and see if there is a supply; then see if the shut-off valve in the line leading to the carburetor is open; if so, drain the bowl of the carburetor to get rid of water or possible dirt. To check the possible clogging of the gasoline pipe, or carburetor screen, notice if the bowl fills up again in a reasonable time.
Do not adjust the carburetor. If the engine has been running, it is practically certain that the carburetor has not gotten out of adjustment. Inspect the intake pipe, or manifold, to see if it has been loosened by vibration. If the engine still refuses to run, put about a tablespoonful of gasoline in each cylinder and crank over the engine. If this runs the engine for a few revolutions, it indicates that the trouble is in the gasoline system and leaves but the spray nozzle, which may have dirt lodged in it, or the auxiliary air valve stuck, as the remaining causes of trouble. Sometimes turning the needle valve a full turn and then turning it back exactly where it was will remove an obstruction at that point. Care should be taken in this to get the needle valve set as it originally was.
Next inspect the ignition system. The first thing to do is to loosen one of the wires from a spark plug and lay it so the bare end will be 1⁄8 inch from the base of the plug, and have someone crank the engine by hand or with the starter. If a spark does not occur, go first to the interrupter points and short circuit the fixed point with a screw driver or other metal tool and see if there is a spark when the engine is cranked. Examine the points for dirt and see if they come together and open properly. Then examine the condition of the battery, testing it. Examine the connectors on the battery, which sometimes jar loose; examine the wires leading to the interrupter and switch; see if they are loose or broken or short-circuited. This need not be done if a spark shows at the interrupter.
Examine the distributor for moisture or dirt and see if the wires have become loose. If a magneto only is used, it is a simple matter to see if the interrupter points are making and breaking properly and if the distributor is clean and dry. If these appear to be all right the trouble doubtless is in the armature winding or the condenser and cannot be repaired upon the road.
The wires to the plugs may be burned or short-circuited. If, with an apparently good spark, you have compression and there is mixture passing into the cylinders, the trouble may be in the spark plug. To test a spark plug it is necessary to remove it. Widen the gap to 1⁄8 inch and lay the plug with wire attached upon the cylinder; crank the engine and see if a spark jumps the gap. Widening the gap is necessary because the spark will not jump so far under compression as in the open air. If it does not jump, the plug may have a broken insulator porcelain or need cleaning. If uncertain about the condition of the plug, exchange it with one in another cylinder which is working properly. An extra set of plugs should be carried to replace those which become dirty; cleaning should be done in the garage.
This covers most of the usual troubles experienced on the road. There are, of course, a great many other possibilities, but if these tests, carefully made, do not disclose the cause of the trouble, the novice had better send for a garage man. If one or two cylinders miss fire, the trouble is most likely to be caused through lack of compression or a short-circuited spark plug. If the engine refuses to run, the trouble is most likely to be due to lack of gasoline, or failure of battery or magneto.