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bon when removed, it should be thoroughly cleaned, care being taken not to scratch the glazing on the porcelain surface, as if this glaze is destroyed it will be possible for the porcelain to absorb oil. The interior of the plug body and the electrodes should also be scraped clean of all carbonaceous matter. If the porcelain is scratched or defaced in any manner it should be replaced with a new one. If the plug is apparently in good condition and yet the cylinder refuses to fire, it may be well to substitute the plug with one known to be in good condition, as there may be some minute short circuit in the porcelain that is not apparent upon inspection.

Plugs using mica insulation are very deceptive, as in many cases short circuits exist that cannot be detected by the eye in daylight. A good way to test a suspected mica plug is to lay it on top of the cylinder after dark, taking care not to have the insulated terminal in contact with any metal parts except the high tension current lead. The engine is then run on the other cylinders and the inside of the spark plug watched to see if sparks jump between the insulator and the plug body, instead of between the points. If a short circuit exists it will be easily detected by the minute sparks plainly evident in the darkness. It is sometimes possible to test a plug out in daytime by shading it from the light in some manner, as with a black felt hat. After the spark points have been set correctly, it is well to double up a piece of emery cloth with the abrasive surface on the outside, as shown at Fig. 82, E, and move it back and forth between the plug points a number of times to brighten them up and to insure that there will be no foreign matter present between them that is apt to short circuit the current. An old tooth-brush and gasoline are the best tools for cleaning a spark plug without taking it entirely apart as stiff brush bristles will remove any oil or material soluble in gasoline. Acetone is a solvent for carbon, and if that material is not baked on too hard it is possible to remove the deposit without scraping it off.

Many cases of ignition trouble have been traced to the use of improper spark plugs or to faulty location of these members. Manufacturers of spark plugs have given the matter of location considerable thought during recent years, and the endeavor is to produce a plug specially designed or adapted for the motor for, which it is to be used. The spark plug shell or base is constructed so the spark points will project into the combustion chamber. It is also important to make provision for proper cooling of the spark plug. This last named factor is an important one that is seldom given consideration by owners or repairmen who change the spark plugs without making sure that they are adapted to the motor. To obtain the greatest efficiency from the explosion it is important that the spark points project into the combustion chamber in such a way that they be surrounded with cool fresh gas. If the gas of the plug is located in a recess for pocket, as indicated at Fig. 52, A, dead gas is apt to accumulate about the points, and combustion will be much slower than it would be with the spark plug located as at B. It will be evident that with this construction of the valve cap the spark points project into the induction chamber, permitting the spark to take place in fresh mixture and promote rapid spread of the ignition flame. Another faulty mounting when a plug is located directly in the combustion chamber is shown at C. It will be apparent that with a projection from the plug body having a space around it in which the hot gases may collect, the plug will heat up much quicker than the mounting shown at D in which the heat will be conducted away by the cooling water. A plug that becomes heated will tend to, soot up and carbonize much quicker than one in which provisions have been made for proper cooling.

Induction Coil Faults.—The high-tension induction coil is one part of the ignition system that can seldom be repaired outside of the factory. In the first place it is not possible to reach the interior parts of an induction coil because the windings and condenser are usually imbedded in a hard insulating compound that has been poured into the coil box in a molten condition, and which becomes as hard as stone when it sets. The only part of an induction coil that is possible to correct is faulty vibrator action, and fortunately the vibrator is about the only part of a well-made coil that demands attention. If the vibrator does not buzz when the circuit is closed at the timer and the wire leading from the timer to the coil unit is found in good condition, the

trouble is due to a broken connection inside of the coil box or the contact points do not touch. If the vibrator operates as it should and there is an extremely bright spark between the points and a weakened secondary spark, it is reasonable to assume that the condenser inside of the coil box is ruptured.

If there is a proper vibration or buzz at the vibrator and no secondary spark from the high-tension terminal, the trouble is either a broken high-tension connection or a short circuited secondary winding. Sometimes a wire inside of a coil is twisted off where it fastens to the terminal screw, due to that member being turned around several revolutions with a pair of pliers. A case of this kind may be fixed by removing the bottom or top of the coil box, as the case may be, and making sure that the connection is resoldered to the terminal post. A punctured winding or short circuited condenser can only be repaired by the coil manufacturer, and in most cases it is cheaper to procure a new coil unit, which is easily removed in modern coils, than to attempt to have the old one repaired.

When a coil unit is suspected of being defective it is easy to ascertain if this is the case by changing it for one of the coil units which is known to be in good condition. If the cylinder which was formerly served by the good coil unit now begins to skip, one may assume that the coil unit is at fault. If the trouble has not been due to other causes, the cylinder that was formerly at fault will begin to operate as it should as soon as the spark plug is connected to the good coil unit which has been substituted for the one thought to be defective.

Adjusting Coil Vibrators. The repairman who understands the vibrating spark coil is the exception rather than the rule. Many are able to adjust a vibrator, but do not know how to locate troubles, or to remove the exposed component such as the bridge, vibrating spring, etc., and reassemble the parts correctly. If the vibrator buzzes weakly when contact is made at the timer, the first thing to do is to test the battery to make sure that there is sufficient current available to operate the vibrator, then the contact points should be examined to see that they are clean and smooth. Various defective conditions are shown at Fig. 83, A; any one of these will interfere with correct contact and with proper vibrator action. At A-l a pit has been burnt in the lower point and a projection has been built up on the upper one. At A-2 the points have been cleaned with a file which has been inserted at an angle so the contact members do not have a true flat surface. At A-3 a point has been built up on one side of the contact of both vibrator springs and contact screw points. As these contact points

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Fig. 83.—Methods of Cleaning Induction Coil Vibrator Contact Screws.

are of platinum it is important to remove as little of that valuable material (which is now worth more than gold) as possible.

For this reason it will be desirable for a repairman working on cars using vibrator coils to provide himself with the simple fixture shown at Fig. 83, B, which insures that the points will be dressed true without removing much material. The fixture is a simple U-shaped piece of hardened steel having a series of holes, A, B, C, drilled into it of such size as will permit the insertion of the most commonly used sizes of vibrator adjusting screws. These are not threaded, the screw F being a free fit in the hole corresponding to the outside diameter of the thread. A feed screw E may be interposed under the adjusting screw in order to feed it up against the smooth file used to clean off the roughness. This screw may be shifted into any one of the tapped poles under the holes A, B and C for feeding different sized contact screws.

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The conventional vibrator is shown at Fig. 83, C, and another form at Fig. 84. It will be noticed that this consists of a vibrator spring or armature carrying one contact point and a bridge member over it carrying another contact which is set into a knurled head adjusting screw in that at Fig. 83, c. The smaller bridge holds the vibrator spring and is also provided with a knurled screw so the vibrator spring tension may be adjusted. Directly under the vibrator is the iron core which attracts it to break the contact between the points. The farther away the vibrator is from

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