If a gasoline engine fails to stop when the ignition switch is turned to the OFF position, the trouble is usually caused by a faulty ignition circuit, improper timing, the octane rating number of the fuel being too low for the design of the engine, or the engine being overheated.

In a magneto-type ignition system, an open ground connection may cause an engine to run after the ignition switch is turned off. When a magneto ground connection is open, the magneto will continue to produce sparks as long as the magneto armature magnets rotate, and the engine will continue to run. In other words, when the magneto ignition switch contact points are closed, the ignition should be SHUT OFF. This is not true of the booster coil circuit of a magneto-type system, nor of the usual battery-type ignition system. In these systems, an open ground or open switch points prevent current flow. If the switch of a battery-type ignition system fails to stop the engine, the contact switch points have probably remained closed.

If the ignition switch and the circuit are in good condition, failure to stop may be caused by overheating. If the engine is overheated, normal compression temperature may become high enough to ignite the fuel mixture even though no spark is being produced in the cylinders. When this happens in a gasoline engine, the engine is, in reality, operating on the diesel principle. Normally, you will detect the symptoms of overheating before the temperature gets too high. The causes of overheating in a gasoline engine are much the same as those for a diesel engine.

Other troubles and their symptoms, causes, and corrections that may occur in a gasoline engine are similar to those found in a diesel engine. Troubles leading to the loss of rpm, irregular operation, unusual noises, abnormal instrument indications, and excessive consumption or contamination of the lube oil, fuel, or water can usually be handled in the same way for gasoline and diesel engines. Of course, there are always exceptions, so it is best to consult the manufacturers technical manual.

Most gasoline engines in the Navy are used by shore activities. Afloat, gasoline engines are used to drive portable pumps like the P-250, a piece of fire-fighting and dewatering equipment. Although pumps like the P-250 are primarily maintained by members of the Damage Controlman (DC) rating, Enginemen are involved to some extent in repairing or overhauling the P-250.

Before you disassemble a P-250 for repair, make sure that all the repair parts are available and on hand. When repairs are not within your ships force capability, you must turn the unit in to an IMA or SRF for repair. Attach an OPNAV 4790/2K (work order form) to the pump. Figure 3-45 illustrates a typical P-250 pump unit.

Figure 3-45.A typical P-250 pump unit.

For more detailed information concerning operation, maintenance and repair of the P-250, refer to the NAVSEA technical manual, Firefighter Pump P-250 Mod 1, S6225-BW-MM0-010.

Some gasoline engines serve as outboard motors to power small boats. A high percentage of the motors problems are electrical. A large number of problems are also caused by the use of fuel with a lower octane content than specified by the manufacturer. To gain knowledge about operating, maintaining, and repairing outboard motors, review manufacturers service manuals and assigned PMS publications. Most outboard motor manufacturers offer a high quality training course, free of charge.