Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(96) Care of Spark Plug.

Porcelains are often broken by screwing the plug too tightly in a cold cylinder, as the cylinder expands when heated and crushes the frail plug. A plug installed in this manner is difficult to remove as the expanded walls grip the thread. The plug should be screwed in just enough to prevent the leakage of gas. A short thin wrench should be used in screwing the plug home such as a bicycle wrench. A wrench of this type is so short that it will be almost impossible to exert too much force, and will be thin enough to avoid any possible injury to the packing nut. Bad leaks may be detected by a hissing sound that is in step with the speed of the engine, small leaks may be detected by pouring a few drops of water around the joint. If a leak exists bubbles will pass up through the water and show its location.

Plugs are more easily removed from a cold cylinder than a hot. If the plug sticks when the engine is cold and is impossible to remove with a moderate pressure on the wrench squirt a few drops of kerosene around the threads. Never exert any force on the porcelain or insulation. The high tension cables should be connected to the plugs by means of some type of “Snap Terminal,” such terminals may be had from automobile dealers.

These terminals make a firm contact with the plug and do not jar loose from the plug by the vibration of the engine. They are easily disconnected when the inspection of the plug becomes necessary, and are generally a most desirable attachment.

The high tension cable should be firmly connected to the plug terminal under all circumstances. A loose connection will cause misfiring or will bring the engine to an abrupt halt. If snap terminals are not used the plug binding screw should be screwed down tightly on the wire. When making connections see that the wire is bright and clean, and that frayed ends of the wire do not project beyond the plug and make contact with other parts of the engine.

A large percentage of high tension ignition troubles are due to short circuits in the spark plug which are generally caused by deposits on the surface of the plug insulation. Soot or oil may be removed from the plug by scrubbing the porcelain and the interior of the chamber with gasoline applied by a tooth brush. Examine the plug for cracks, and if any are found, replace the porcelain or throw the plug away. A cracked porcelain is always a cause of trouble.

To test a plug for short circuits, remove it from the cylinder, reconnect the wire, and lay the sleeve of the plug on some bright metal part of the engine in such a way that only the threaded portion is in contact with the metal of the engine. Close the switch and see if sparks pass through the gap. If no sparks appear, and if the coil is operating properly, clean the plug. As an additional test for the condition of the coil, hold the end of the high tension cable about ¼ inch from the metal of the engine while the coil is operating. If a heavy discharge of sparks takes place between the end of the cable and the metal of the engine, the coil is in good condition.

If a partial short circuit exists, the spark at the gap will be weak and without heat; the result will be intermittent, or misfiring with a loss of power. Moisture in the cylinder is a common cause of plug short circuits, the moisture coming from leaks in the water jacket or from the condensation of gases in a cold cylinder. A drop of water may bridge the spark gap, allowing the current to flow from one electrode to the other without causing a spark.

If a cloud of bluish white smoke has been issuing from the exhaust pipe before the misfiring started, you will probably find that the trouble is due to sooted or short circuited plug.

The remedy is to decrease the amount of lubricating oil fed to the cylinder.

When a magneto is used the intense heat of the spark causes minute particles of metal to be torn from the electrodes and deposited on the insulation as a fine metallic dust. This will of course cause a short circuit and must be removed. Short circuits are sometimes caused by the magneto current melting the electrodes and dropping small beads of the metal between the conductors. All metallic particles should be removed from the plug.

While a spark plug may show a fair spark in the open air test, it will not always produce a satisfactory spark in the cylinder on account of the increased resistance of the spark gap due to compression.

Compression increases the resistance of the spark gap enormously and thin, highly resisting carbon films that would cause very little leakage in the open air will entirely short circuit the gap under high pressure, the current taking the easiest path which in the latter case is the carbon deposit.

In order to produce conditions in the open air test similar to those in the cylinder we must devise some method of increasing the resistance of the spark gap in the open air above any possible resistance that could be offered by the carbon film.

Placing a sheet of mica or hard rubber between the electrodes, or in the spark gap, will increase the resistance to the required degree. If the spark plug is in good condition the spark will jump from the insulated terminal to the shell when the mica is in the spark gap, but if a short circuit exists the current will go through it without causing a spark. It is assumed that the battery and coil are in good condition when making the above test.

If the electrodes or spark points are dirty they should be cleaned with fine sand paper, special attention being paid to the surfaces from which the spark issues. When reassembling the plug, see that all of the washers and gaskets are replaced and that the length of the spark gap is unchanged. A little change in the spark gap may make a great change in the spark. A good spark is blue white with a faint reddish flame surrounding it. When the discharge is intermittent or sputters in all directions, either the coil or the plug are partially short circuited. Always have a spare plug on hand.

Ordinarily the length of the gap or the distance between the electrodes should be about 1
32 inch for batteries, and a trifle less for magnetos. A silver dime is a good gauge for the gap. If the engine misfires with the coil and batteries in good condition, try the effects of shortening the gap a trifle, usually this will remedy the difficulty. Exhausted batteries may be made operative temporarily by closing up the plug gap to 1
64 inch or even less. Shortening the gap increases the heat of the spark and nothing is gained by having it over 1
32 inch.

Almost all high tension magnetos have visible safety spark gaps that show instantly the presence of an open circuit in the secondary or high tension circuit. If an open circuit exists, a stream of sparks will flow across the safety spark gap at low speed.

To determine the cylinder that is misfiring in a four cylinder engine proceed as follows:

Remove cover on spark coil, and hold down one vibrator spring firmly against the core while the engine is running.

If the engine speed is not decreased by cutting this coil out of action, it is probable that this is the coil connected to the misfiring cylinder. Now release this vibrator and proceed to the next coil, and hold its vibrator down. If this decreases the speed of the engine you may be sure that the first coil is in the defective circuit. If the vibrator buzzes on the coil under inspection the trouble will be found in the plug.

Cutting out a coil connected to an active cylinder decreases the speed of the engine. Cutting out the coil connected with a dead cylinder makes no difference.