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Magneto Information


The magneto is a self generating ignition system. Inside there is an armature and field magnets just like a generator. The armature/field generates the low voltage required by the internal coil that steps up the voltage for the spark plugs. The coil, points and condenser and rotor are all inside the magneto and operate without any outside wires or voltage. The only wire required is to short out this system to stop the engine. The magneto also has an impulse start mechanism that stops the armature from turning momentarily and then clicks loudly when it lets go. This does two things. One it retards the spark so it does not kick back and two, when it lets go, the armature spins fast enough to generate the low voltage required for getting a spark when hand cranked very slow.

The rotor must point to this timing lug when timing the magneto

                     Timing a mag. (FMJ)
1. If you have taken the Mag. apart, the internal gears need to be timed.  (Set points at .020)
2. Remove the large cap and line the C mark on the fiber gear with the  marked tooth on the steel gear. Tip- turn the engine backwards to get the steel gear tooth to stop straight up and position  the C mark on the fiber gear straight down. This times the rotor to the points. It is pretty simple in theory. Like all four cycles, the piston comes to TDC (top dead center) twice for every time the magneto or distributor rotor goes around. ("B"&"C")The left side of the engine has a hole in the bell housing where you can see the timing mark on the flywheel.  ("WD" &" WD45"  The timing mark is found by looking under the flywheel housing for a small hole possibly covered by a small plate held with two wing bolts. The FIRE mark and the TDC mark are located on the flywheel through this hole. Use a little light colored paint to make seeing them much easier). 
3. Take out the #1 spark plug, and put your thumb over the hole while hand cranking the engine(best to have all the plug wires off while doing this so the tractor doesn't try to fire) , the cylinder will force air past your finger on the compression stroke. (The other FIRE is the exhaust stroke and has the exhaust valves open so no air will come out of  the plug hole).
4. When you feel the air coming past your thumb, look for the FIRE mark on the flywheel. When this mark is centered in the hole, the rotor on the mag should be just lower than the #1 lug on the mag. Looking at it as a clock, #1 lug should be near 10:00. Just before the lug is a plastic stud called the timing stud. Perfect timing is when the rotor points to the timing stud while the FIRE mark on the flywheel is centered in the hole.
5. If your rotor points to 5:00 your mag is 180 degrees off and needs to be removed and the coupling turned till you achieve 10:00. If it is not right on the timing stud, you can loosen the mag and rotate it until it is.
6. Put it all back together and time your spark plug wires 1-2-4-3 clockwise on the magneto cap.  

  Remember if the rotor is aligned with the timing lug when the fire mark is in the window, you got it right. If it's not quite aligned when the fire mark is in the hole, then you rotate the mag clockwise or counter clockwise until it is. Remember  Perfect timing is when the rotor points to the timing stud while the FIRE mark on the flywheel is centered in the hole.


Fiber Gear in Mag Cap

 

Steel Beveled Gear in Mag

 

Rotor Position to Timing Lug for Meshing Fiber Gear to Beveled Tooth on Steel Gear

If you are missing the mark ("C") on the fiber gear, align the rotor to the timing lug as shown and with the beveled tooth straight up, install the cap.


CHECKING OUT A MAGNETO WITH NO SPARK
Here are a few things to check out. First remove the wire from the switch at the mag (could be a bad switch-also check the little strap on this connection, if there is one, and see that it is not grounded) and see if you now have spark.

If not, next remove the mag and clamp the flange in a vise as this easier than cranking and stretching etc. Remove the 4 screw cap and very carefully check the points. (Even a oily fingerprint can stop the low voltage from going through the points.) Clamp a visegrip on the drive lug to turn it over. When the impulse lets go, you should see a spark between the points.

If you don't, you are either losing the spark to ground, like the condenser connection touching or a field wire shorted out or the points are not adjusted to .020 gap or they are burned or dirty. (The points actually ground the spark, generated by the armature and fields through the coil. This low voltage passing through the coil, causes an induced high voltage surge that goes to the sparkplug.)

Soooo, if you have the low voltage timed right and the rotor is at the correct lug, at the correct time with the camshaft, you have "Fire in the Hole".

Now, if you have the spark at the points and don't have spark at the plug, when everything is assembled, it could be out of time, a bad coil or condenser, possibly a cracked cap, or shorted out rotor. A shorted cap or rotor can have what appears to be a crack where the spark goes to ground. 

I hope I have enlightened you to some of the workings of a magneto and this helps you resolve the problem. Let me know. I always like to hear back.
Duey