Timing Magnetos

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Timing the Mags on your engine

What you will need:

1.       Straight edge

2.       New mag gaskets, 

3.       “Never seize”.

4.       Plug wrench 

5.       15/16” open end wrench, 

6.       Screw driver 

7.       1/2’ socket or suitable to loosen the nuts holding the mag in place

8.       Mag timing light; borrow one or better yet take the plunge and spend $42.95 for the Action Air Parts Inc., Inductor Dual Mag Synchronizer (P/N 12-16901 from Spruce), since you should check the timing each annual it will quickly pay for itself.


Removethe upper cowl, and if you have split your nose bowl, remove the top half as well. (If you have not split the nose bowl you may want to think about is, unless you are a purist or Luddite, splitting the bowl makes access especially easy. I will cover this in a future Technotes. 

Loosen the mag retaining nuts just enough to be able to advance or retard the timing by moving the mags in the slotted flange. If your mags are stuck tight, you will have to pry/tap them loose and replace the gaskets well coated with Never Seize. (If you do this, read Installing Mags below) Remove the top plugs, rotate the engine in the normal direction until you have TDC on #1 cylinder. You will hear the impulse couplings go off as you approach this position

(Place your finger over the spark plug hole and rotate until you feel pressure build to maximum). There is a special tool to find TDC that you really don’t need. On a 220 you can see the piston travel and verify the position by looking in the sparkplug hole. On all engines there is also a mark on the prop flange showing 28°, 32° and TDC. Connect the timing light by putting the green lead on the left mag, and the red lead on the right mag, you will note a thick braided wire exiting the bottom of the backplate of the mag, this goes to the noise suppressor, also attached to the mag. A wire is screwed into the suppressor that disappears into the firewall; this is the famous “P” Lead.(Connect the green and red alligator clips to the screw that attaches the P lead to the noise suppressor). Black lead to airframe ground, turn the ignition switch to BOTH and switch the timer ON.

 On Franklin’s both mags have impulse couplers. Now rotate the prop backward about 1/8th of a turn, place the straight edge along the engine crankcase joint, see diagram 1, and then slowly rotate the prop fwd. To the timing mark for your engine. (150 are 28°, 165 is 32°, 220 is 28°). On the timer both lights will be ON, as you arrive before, at or just past the timing mark the red and green lights will go out. Carefully align the straight edge with the XX° mark, do this by moving the prop in the normal direction of rotation to the mark. Do not jiggle back and forth on the prop, as there is a lot of lash in the drive gears and you will take up the slack only and so the engine will not be in the correct position. If you had to remove the mags and replace the gaskets, which I think is a good idea any way….

Installing the Mags

 With the engine set at the XX° position, remove the little plastic screw cap on the top of the mag, and rotate the drive gear until you see a red marked tooth inside the hole that you removed the plastic cover from.  The mag will rotate freely in one direction and in the other it will come up against the advance coupling, turn the easy way. If you want to test the advance coupling make sure you have not got your finger in one of the plug lead holes! With the gaskets in place and well lubed with that sticky silver stuff that gets everywhere and is a pain to remove, insert the mags into the drive holes, keeping the little red tooth exactly in the center of the inspection hole, and gently finger tighten in place.

Setting the timing

 With the straight edge and the XX° mark aligned, the green light on the left mag should go out and there should be a continuous beeping sound emitting from the timer. If this is so, tighten left the mag in place. Turn the right mag in the slotted hole until the red light goes out and the pipping sound becomes a continuous whine. What you are trying to do here is have both green and red lights out and a continuous whine from the timing box, when the engine is in the XX° BTDC position. That’s all there is to it.

See also pages 33 to 39  in service manual.