Piper Comanche PA-24 Service Manual page 548

a. Turn the engine crankshaft to number one cylinder full advance firing position. (Refer to Figure 7B-13.)
Install the magneto on the engine and tighten the mounting clamps sufficiently to hold magneto
position and yet allow it to be rotated.
NOTE: Verify engine timing for the airplane being worked on by checking the engine dataplate.
CAUTION: EXTREME CAUTION MUST BE EXERCISED WHEN ALIGNING ONE OF THE
PAINTED LINES ON THE DISTRIBUTOR FINGER WITH THE PAINTED LINE
ON DISTRIBUTOR BLOCK. AFTER MAGNETO HAS BEEN INSTALLED ON
ENGINE, CHECK THROUGH OPENING MARKED "T" TO SEE THAT THESE
MARKS (LINES) ARE STILL PROPERLY ALIGNED. CAM LOBES ARE SO
CLOSE TOGETHER THAT IF THE MAGNETO IS TURNED ONLY SLIGHTLY IT
WILL BE POSSIBLE TO TIME MAGNETO ON THE WRONG CYLINDER. IT
COULD BE ON THE NEXT CYLINDER IN FIRING ORDER OR IT COULD BE
ON PREVIOUS CYLINDER IN FIRING ORDER. IO-720 SERIES ENGINE
FIRING ORDER IS 1-5-8-3-2-6-7-4.
b. Connect a timing light to switch wire (shielded capacitor wire) and to a good ground on engine.
c. If the timing light is out (battery operated models), rotate the magneto housing in direction of its
magneto rotation a few degrees until light comes on. Then turn slowly in opposite direction until
light goes out. Secure the magneto in this position.
NOTE: The magnetic force in the S-700 series magnetos tends to act in the direction in which the
rotor is turned during the timing operation. This causes the backlash in the magneto drive
gears to accumulate in the same direction. Consequently,. as the magneto is being timed,
the backlash can be o\erlooked and result in a timing error as much as 6 to 8 degrees. To
eliminate this possibility, insert a small screwdriver through the "T" marked opening and
engage the painted rotor timing groove. (The painted lines on the dstributor block and
finger will be found in approximate alignment.) With a light force depress the screwdriver,
moving the rotor in a counterclockwise direction, as viewed from the rear, sufficiently to
remove any backlash while at the same time, rotate the magneto back and forth in both
directions until the breaker points just begin to open, as indicated by the timing light. If the
"T"-marked hole is inaccessible, preventing it from being used for holding out backlash,
remove the magneto distributor block and use the magneto rotor for this purpose.
d. Repeat this procedure for the other magneto using another timing light.
e. After both magnetos have been satisfactorily timed, leave the timing light wires connected and
recheck timing as previously described to make certain that both magnetos are set to fire
simultaneously. If
VIIB - POWER PLANT
(PA-24-400)
PIPER COMANCHE SERVICE MANUAL
2F18
04/01/09