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AIRPLANE MOTORS.       75

missing or the left side? Notice which cylinder is giving the trouble. Knowing which cylinder is giving trouble, we can find the trouble comparatively quickly. Some kinds of trouble can cause the entire motor to give trouble, but could not cause just one cylinder to give trouble. These could be called "general" troubles. Another kind can cause one or two cylinders to miss or give trouble but could not cause a general trouble. These we call "local" troubles.

3. For example, suppose we say our motor is missing. Now, in addition to noticing at what speed the motor is missing, let us note the nature of the miss. For instance, is one cylinder missing occasionally, one cylinder missing steadily one side of the motor cutting out, or the whole engine cutting out, as it would do if we intermittently pulled the switch? Or is it a scattering miss, which means first one cylinder on the other side missing, but not regularly? Suppose we decide that we have one cylinder missing occasionally. Some of the likely causes of this trouble would be trouble from oil, slightly cracked porcelain in spark plug, parts binding in the valve action or need of adjustment in the valve clearance; it might be after on the spark plug. Suppose we decide that we have one cylinder missing steadily. This is likely to be caused by one of the secondary wires or leads, running from the distributor to the spark plug, being disconnected at either end; a broken spark plug, or a burned exhaust-valve spring, etc. Suppose we decide that one side of the motor is giving trouble. If this trouble occurs at high speed, it is most likely to be caused by magneto troubles (this can sometimes be remedied by slightly retarding the magneto) or it may be an inlet valve stuck open. If the trouble occurs at low speed, it is pretty sure to be caused by an air leak in the inlet manifold on this side of the engine, a weak exhaust valve spring, which has the same effect, or possibly an exhaust valve held or stuck open.

4. Magnetos will cause one side of a "V' type motor, or every alternate cylinder in a six-cylinder motor, to cut out at high speed, and this is sometimes caused by the breaker points opening a different distance on the two cams; sometimes by the weakening of the magnets, due to vibration or other cause and sometimes by wear in the armature bearings.