If the engine misses at high speed, close the points a little to make the opening come later and when in better armature position, the action being just the reverse of the foregoing paragraph.
Widening the gap of the spark plug also will help to overcome skipping at low speeds. At low speeds a full charge of gas is not drawn in and compression is low and temperature low in consequence; the mixture is not so near the self-ignition point when the spark occurs as it is at higher speeds. Therefore a better spark is needed to ignite it. By widening the gap in the plugs the secondary current is held back until it acquires high enough potentiality to jump the greater gap; so that when it does occur an exceptionally powerful spark is obtained.
This may be demonstrated with the use of a vibrating coil. Set one of the cylinders on top of compression ready to ignite. Turn on the current and while the vibrator will buzz the spark inside the cylinder is not strong enough to ignite the charge. Now detach the spark-plug wire and hold it close to the outer plug terminal and turn on the current. The extra gap will increase the strength of the spark in that cylinder and it will ignite the charge, starting the engine. This has led to the use in some cases, of outside spark gap attachments.
Widening the gap and getting a longer spark is not what ignites the charge. It is simply that the greater gap causes the current to build up to a higher pressure, or voltage, before it will jump the gap, so that when it does occur it is of greater intensity.
There are several other things about sparking not generally understood by the novice. The general opinion is that the greater insulating surface of a plug there is inside the cylinder the less likelihood there is of short circuiting and consequently the more certain will be the spark occurrence. This is only partly true; it is true so far as short-circuiting is concerned; but where the surface is too great and soot or carbon is deposited upon the insulator surface, the carbon acts as a condenser and absorbs the secondary current. The effect is that it will not build up a sufficiently high voltage to jump the gap. Very heavy insulation on the secondary wires absorbs current in the same way.
The fact that the spark jumps across the points of the plug when it is in the open air does not necessarily mean that it jumps when the points are under compression in running conditions. A current which will cause a spark to jump a half-inch gap in the open air, will cause it to jump a gap of only 1⁄32 inch under normal running compression. Failure to appreciate this often results in a man thinking his ignition is all right when there may be a crack in the insulation so small that it scarcely can be seen with the naked eye when cleaning the plug, or there may be a porous spot in the porcelain, either of which would leak current under compression. Many manufacturers test the porcelains to see if they leak, using a spark gap of 3⁄4 inch under a very high voltage.
If there is carbon on the porcelain, the current very often will travel through this under high compression, because there is less resistance offered by the carbon than by the spark gap. It short-circuits the plug and there is no ignition. Yet, when this same plug is removed and tested in the open air, there will be a good spark; the resistance of the gap in the open air would be less than the resistance offered by a path through the carbon coating. If on testing a plug in this way there seems to be a good spark, yet the cylinder will not fire, clean the plug thoroughly and try it out in the cylinder. If it still does not fire, try a plug which you know is all right and see if it fires. If it does, you may know that the old plug leaks current somehow and probably needs a new porcelain.
It is a common error with motorists when cleaning a spark plug to polish up the points and call it a job. What really is required is to remove all of the carbon, which is a fair conductor of electricity, from the porcelain or mica and get these back into the insulator class again.
The conditions which allow the adjustment of the interrupter points in magneto ignition to accommodate certain speeds, do not arise when battery ignition is used. Battery interrupter points are usually adjusted for an opening of 1⁄64 inch and the gap at the spark-plug points must be greater usually than when magneto ignition is used, because the spark given by battery systems is naturally of less volume than that given by the magneto. Therefore the gap must be greater to intensify the current as before described.
The interrupter points sometimes become coated with an insulating glaze, composed of oxides and dirt hammered by the action of the movable interrupter point, or hammer. The points will have the appearance of being all right; there will be no pits or points; nevertheless there may be enough insulating material there to keep the current from flowing and no spark will occur in consequence. The points may be cleaned by inserting a very thin file between the points when they are open and then allowing them to close on the file, being held together with no more pressure than the spring exerts, and working the file back and forth until the glaze is removed. It is necessary to have a flat surface, so that care in using the file is needed, and, as platinum is more valuable than gold at present, it will be apparent that as little filing as possible should be done.