Mag timing on a JD 40 A

If you remove the plugs for easy flywheel turning, you can rotate the flywheel so that the left (flywheel side) cylinder is at TDC on its compression power stroke, and at that time the Left Hand Impulse mark on the Flywheel should be at the 3 o clock position in line with the flat horizontal mark on the side of the tractor. Then, the female drive cog slot in the end of the governor shaft (that drives the mag) should be flat horizontal. On a Wico C or X Mag, if you then rotate it to where the first leading rotor tip is before and left of (looking in with cap removed) where the caps top plug terminal is, thats where it would be ready to snap over and fire the left plug at TDC out the top terminal, just when the LH Impulse on the flywheel passes that 3 o clock position. To install the mag there, however, you get it rotated where the leadign brush tip is just before and left of the caps top terminal, and then turn the flywheel backwards so the mags male slot fits into the governor shafts female slot.

After the mag is in with its bolts still loose, to set the initial start timing, you rotate it CCW to advance and CW to retard. You should hear it snap to fire approximately when the LH Impulse mark on the flywheel is right at the 3 o clock mark however. If it fires too much before then, the flywheel may kick back. I set them by having the plugs out and turnign the flywheel and seeign where that LHI flywheel mark is when you hear the mag snap. Again, you want her to snap just when the LHI mark passes that 3 o clock position mark on the tractors side.

Now, other mags like Fairbanks Morse fire only at TDC on the power stroke and not the compression. On those, they have to be timed so the correct plug fires out the correct (left or right intended) terminals. Still, by checking which cylinder is ready to fire at TDC on its power stroke, you can get it in right with the correct wires to the corect (left or right) cylinders.

The above addresses initial start timing, while the run timing advance is set by where the index marked plates on the rear of the mag are set. On some, 2 1/2 marks to the left equals 25 degrees of runnign advance. Thats different on different mags and different couplers, but it may well be fine where it is, as those dont usually get changed all that much.

Hope this gets you going, if you dont want to worry with all the above, just stick her in by aligning the governor shaft and mags drive cog/slots, and you have a 50/50 chance of beign right, and can swap top n bottom wires or pull her out and rotate the flywheel 180 and put her right back if its a Wico C or X and its not that much more work even on others that have the simple slotted drive cog. The fine tune it by mag rotation.

Best Wishes n God Bless

Ol John T Nordhoff in Indiana

~To install the mag & time it to the tractor, [this will have to be general due to not knowing mag type]. Remove the L/H spark plug & turn the engine over until you feel compression at the plug hole, then keep turning it until the L/H impulse writing on the flywheel lines up with the case mark [at 3 o'clock on the case behind the flywheel] . At this time the mag drive flange on the R/H end of the governor shaft should have the slots facing fore & aft [parallel with the ground].....Then install a wire or paper clip in the front [or top] mag cap plug wire tower & bend it within 1/8" of the metal mag housing, then while holding the mag as it is installed in the engine turn the mag drive cup CCW [to the left] until it snaps & you see a spark between the wire & mag case, then back the drive cup up slightly until the drive cogs are running fore aft [parallel with the ground] , then install the mag in the tractor.

~To set 'most' Deere "A" 2-cylinder mags so they spark at the correct time... the L/H spark plug should have a spark as the words [L/H impulse] on the flywheel lines up with the tractor case or cover mark [at 3 o'clock behind the flywheel & at the same height as the center of the crankshaft] as the flywheel is turned in the starting direction [CCW]....Or, if you have a late "A", as the punch mark on the flywheel hub lines up with the punch mark in the bottom of the flywheel cover center hole as the flywheel is turned in the starting direction [CCW].

~If you aren't getting a spark at the correct time on the L/H plug wire as the [L/H impulse] lines up with the case mark, [while turning the flywheel in the starting direction], but the spark is within a couple of inches of where it should be, loosen the mag hold down bolts & turn the top of the mag forward to retard the spark or turn the top rearward to advance the spark...If the spark isn't even close, try swapping the plug wires from tower to tower in the mag cap & see if it gets closer. If you still can't get it to spark at the correct place even with the plug wires in either cap tower, turn the flywheel until it's slot in it is running fore & aft [parallel with the ground], then pull the mag & see if the mag drive flange slot on the governor is running the same direction [parallel with the ground], if not you will have to lift the governor & turn the gear match up until it does.

~The flywheel MUST be installed correctly for the above to be of any use, so make sure the flywheel in installed on the crankshaft correctly & the >< marks are matched between the flywheel & crankshaft. If you can't find any marks, or are in doubt about the flywheel position, remove the L/H spark plug & use a pencil or wire to feel for the L/H piston to come to the top of the stroke & at that time the flywheel [L/H impulse] should be at or close to the case mark.

~Not all mags used on 2-cylinder Deere's fire on every flywheel revolution & not all have a solid audible snap, so make sure you are on the L/H compression stroke while setting & looking for a spark.

~The above information should get you running but if you post back with the mag brand & model as well as the number on the impulse cup we can give a little more specific information such as setting the impulse stop ring & total mag running timing ...