times as many turns in the secondary as in the primary, the  induced voltage that results from lines of force linking the  secondary is high enough to furnish ignition for the engine. The condenser, which is connected across the contact points,  has an important function in this circuit.  [Figure 4-25]  As  current flow in the primary coil is interrupted by the opening  of the contact points, the high self-induced voltage that  accompanies each collapse of the primary magnetic field  surges into the condenser. Without a condenser, an arc would  jump across the points with each collapse of the magnetic  field. This would burn and pit the contact points and greatly  reduce the voltage output of the booster coil. The booster  coil generates a pulsating DC in the primary winding that  induces a high-voltage spark in the secondary windings of  the booster coil. Many opposed reciprocating engines are equipped with  an impulse coupling as the auxiliary starting system. An  impulse coupling gives one of the magnetos attached to the  engine, generally the left, a brief acceleration, that produces  an intense spark for starting. This device consists of a cam  and counterweight assembly, spring, and a body assembly.  [Figure 4-26]  The assembled impulse coupling is shown  installed on a typical magneto in  Figure 4-27 . The magneto is flexibly connected through the impulse  coupling by means of the spring so that at low speed  the magneto is temporarily held.  [Figure 4-28]  The counterweight, because of slow rotation, catches on a stud  or stop pins, and the magneto spring is wound as the engine  continues to turn. The engine continues to rotate until the  piston of the cylinder to be fired reaches approximately  a top dead center position. At this point, the magneto  counterweight contacts the body of the impulse coupling  and is released. The spring kicks back to its original position,  resulting in a quick twist of the rotating magnet of the  magneto.  [Figure 4-29]  This, being equivalent to high-speed  magneto rotation, produces a spark that jumps the gap at the  spark plug electrodes. The impulse coupling has performed  two functions: rotating the magneto fast enough to produce  a good spark and retarding the timing of the spark during  the start cycle. After the engine is started and the magneto  reaches a speed at which it furnishes sufficient current, the  counterweights in the impulse coupling fly outward due to  centrifugal force or rapid rotation. This action prevents the  two counterweight coupling members from contact with the  stop pin. That makes it a solid unit, returning the magneto to  a normal timing position relative to the engine. The presence  of an impulse coupling is identified by a sharp clicking noise  as the crankshaft is turned at starter cranking speed past top  center on each cylinder. problem that can arise from impulse couplings is that the  counterweights can become magnetized and not engage the  stop pins. Congealed oil or sludge on the counterweights  during cold weather may produce the same results. This  prevents the  counterweight weights from engaging the stop  pins, which results i no starting spark being produced. Wear  can cause problems  with impulse couplings. They should  be inspected and any maintenance should be performed as  Figure 4-26.  Parts of an impulse coupling.  4-17 Impulse Coupling Body Counterweights Cam Spring