Page:Motors and motor-driving (1902).djvu/193

Rh advance of the firing; and the shorter the distance the more advanced it becomes, the cam stalks in rising covering the reduced distance and breaking contact the sooner. And the tendency is in this direction, for after running a thousand miles or so, small craters become burnt away on the under surface of the ignition plugs, where contact occurs, and the inside arms of the palettes, in rising to fill these, bring down the outside ends and reduce the distance. In practice this is no particular disadvantage, down to about mm., but below or even at that distance, a sharp back-fire may be experienced on the starting-handle, or if the engine be kept running very slow on the throttle valves, the motor may possibly be stopped. This distance should therefore be verified at intervals, and if necessary restored, which is done either by bending upwards the outside arms of the palette with the pliers, or by putting in a new set of ignition plugs, when the craters may be removed from the old ones by filing flat, and turning a corresponding amount off the flanges. This amount rarely exceeds $1⁄2$ mm., and the plugs are then as good as new. Even in this case the palette outside arms have generally to be slightly bent, up or down, as the case may be, and it will save a lot of trouble to adjust the distance at 2 mm., and never to go below 1$1⁄2$ mm. When a new adjustment has been made, the cocks in the cylinder heads should be opened successively, just to see that each cylinder is firing equally and clean.

Should any difficulty be experienced in starting, and should the other parts of the engine be apparently in good order, the following may be looked at for the cause:—

The armature spindle ends may be examined to see that they are clean, and therefore in good contact.

The ignition plugs may be examined to see that they are not coated with oil, which would prevent a proper contact with the rocking arms. This may occasionally happen if too much oil has been allowed to accumulate in the crank chamber.

The little plug-hole on the dash-board, into which a