Back Firing.—I have said that a considerable resistance should always he felt when turning a motor slowly by hand; sometimes, however, the innocent motorist, when endeavouring to put his engine into motion, receives something considerably more than a mere resistance. He may get a kick from the handle which will give his arm a nasty jar, or possibly sprain his wrist. These 'back fires' are the result of what is called 'premature ignition,' and therefore belong strictly speaking to the chapter on 'Ignition.' I may merely remark that they are due to the spark-timing gear being too much advanced, the platinum tubes being too long, the burners being too close in, faulty opening of the induction valves, overheating of the motor, or ignition tubes being too hot; in the last case the burners may be turned down to allow the tubes to cool momentarily and turned up again when the motor starts. In the other cases the remedies are obvious.
4. Moving Parts.—Having exhausted the possible causes of refusal to start except those consisting of some mechanical fault, we will now see what moving parts could go wrong, and so cause all the trouble.
(a) The mechanism for operating the electric ignition is liable to many little derangements (see Chapter VIII.).
(b) A broken exhaust-valve stem or a broken or displaced spring will often be difficult to observe; a valve may have stuck open through stiffness or through something getting under its seat. The exhaust-valve gear should be carefully watched to see that all the valves work regularly and to their full extent.
(c) Stiffness in the accelerator or governing gear, or a dislodged key, pin, or feather may also hinder the lifting of the exhaust valves, or on some engines prevent the throttle from opening itself fully.
(d) The simple mechanism adopted on some cars to engage the starting handle with the motor will sometimes give out, so that the handle will not turn the motor. In cars of the Panhard type the handle is made to engage with the engine by pushing a small bevel-ended tongue in against a small pin put
