Friday, July 20, 2012

Cessna 152 Magneto Timing



         The magneto is equipped with an impulse coupling. 
Internal timing is fixed and the breaker points are not adjustable. 
        Timing marks are provided on the distributor gear and distributor block visible through the air vent holes.
          For timing to the engine a timing hole is provided in the bottom of the magneto adjacent to the magneto flange.                                                                                                                                                                      A timing pin can be inserted through this timing hole into the mating hole in the magneto rotor shaft to lock the magneto approximately in the proper firing position. 
        Rotate the crankshaft in direction of normal rotation until No. 1 cylinder is on the compression stroke and continue rotating the crankshaft until the correct advance timing mark on the front of the starter ring gear is in exact alignment with the small drilled hole located at the two o'clock position on the front face of the starter housing. Leave the crankshaft in this position until the magneto is installed.
NOTE
In all cases, it must be definitely determined that No. 1 cylinder is at the correct firing position, on its compression stroke, when the crankshaft is rotated in its normal direction of rotation.                                After the engine has been placed in the correct firing position. install and time magneto to the engine in the following manner.
a. Remove the timing (vent) plug from the bottom of the magneto. The vent plug in the top of the magneto need not be removed.
b. Rotate magneto shaft until timing mark on rotor is visible through the vent plug hole. Impulse coupling pawls must be depressed to turn magneto shaft in normal direction of rotation.
c. Establish that the magneto is at number one firing position. It is possible for the timing mark to be visible while firing position is 180 degrees from number one firing position. It is necessary to "spark" the magneto to establish the correct firing position. The outlet plate with spark plug leads must be installed on the magneto. Hold number one spark plug lead close to magneto case, or ground the magneto and hold the number one spark plug lead close to a good ground. Rotate impulse coupling in normal direction of rotation until a spark occurs at this lead. (Impulse coupling pawls must be depressed to turn magneto shaft in normal direction of rotation.) Turn impulse coupling backward a few degrees, until timing mark on rotor is centered in vent plug hole and install timing pin (or 0.093 inch 6-penny nail) through hole in bottom of magneto next to the flange and into the mating hole in the rotor shaft. This locks the magneto approximately in firing position while installing it on the engine.
d. Keep timing mark centered in vent plug hole during installation.
e. Be sure magneto gaskets are in place and that engine isin correct firing position, then install magneto(s) at the angle noted during removal, tighten mount nuts finger tight.
f. Connect one lead of timing light to the capacitor terminal at the rear of the magneto and ground lead of timing light to a good ground.
g. Rotate propeller opposite to normal direction of rotation a few degrees (approximately 5 degrees) to close magneto contact points.
Do not rotate propeller back far enough to engage impulse coupling, or propeller will have to be rotated in normal direction of rotation until impulse coupling releases, then backed up a few degrees before the firing position.
h. Slowly advance propeller (tap forward with minute movements as firing position is approached) in normal direction of rotation until timing light indicates position at which contact points break. The contact points should break at the advanced firing position of number one cylinder. Loosen mounting nuts slightly and rotate magneto case to make contact points break at the correct position. Tighten mounting nuts.
To prevent possible condenser damage, do not exceed 13-15 lb-in torque when tightening "P" lead nut during installation of Slick magneto.
i. After tightening magneto mounting nuts, recheck timing. Make sure both magnetos are set to fire at the same time. Remove timing equipment and connect spark plug leads and ignition switch leads.

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