Timing Your Magneto Ignition Tractor
If you have done major engine work or restored your tractor, chances are you removed the magneto and spark plug
wires and eventually reached the point where you had to put it all back together and make it run. On our first
cosmetic restoration, not having a manual, we carefully marked the wires, taped the magneto in the position it
came off, and were careful not to turn the engine over while we had these components off. We thought we could get
by with this since the engine ran perfectly and would not need any internal work. After the cleanup and painting
was done, we began reassembly and finally came to the day that it was time to put the magneto back on and start
it up. That didn't turn out to be the day we would start it! The tape didn't stick too well to the mag body and
was long gone. We bolted it on as near as we could remember to how it came off. We hooked up the wires according
to the diagram we had made. We cranked till we had massive blisters and the best we could get was an occasional
pop. Remember, this was a perfect runner when we took it apart. After much examination and we discovered that our
diagram was showing the wires number left to right with the rear being number 1. This would have been fine (though
technically inaccurate) had the same person hooked up the wires that made the diagram. I had hooked them up with
number 1 being the front plug (which is the case for all tractors I know of but not all engines). This was easy
enough to correct but the magneto had still turned and was not aligned to its drive. We finally were forced to
learn how it was supposed to be done if we wanted to do more than just push the shiny new tractor in and out of
the shop when we wanted to work on something else.
Our approach could have worked but too often you won't have the luxury of trying to keep the components aligned
correctly. Understanding the magneto setup is pretty simple anyway. There are two phases to the process. The first
is to get the magneto roughly timed, the second is to dial it in. The "roughing" process is as follows:
Aligning the Cap
There is a gear on the magneto shaft that will drive a gear on a rotor (similar to a distributor ignition).
These gears must mesh at the proper point. On the FMJ style magnetos, There are 2 teeth with bevels on the magneto
side and a tooth with a timing mark on the cap side. The marked tooth meshes in between the bevel teeth. The intent
of this process (in case all the marks are gone), is to get the points to open and close at the correct times to
provide voltage out the plug wire. Knowing this it is possible to guess the meshing in the case of missing markings.
Once the cap is aligned, it can be screwed back on.
Since the need to do this step means you must have the cap off, it is a good time to check the gasket and make
a new one if it is damaged. A new gasket must be made from very thin paper material, heavy gasket material such
as used at oil-bathed seams can hold the cap out as much 1/16th inch or more too far. Also don't forget to clean
the vent holes at the bottom of the cap.
Lining up the Engine
The engine should be at the top of the compression stroke of the number 1 piston. There were many ways that
this was marked on engines, none of which were standard and in any case, the timing marks are often missing on
old machines. There is a sure fire way to know. I use a small fitting with the same thread as the spark plug on
one side and a nipple for a hose other side. These are commonly made for compression testers and the "head-still-on-engine"
valve spring compressor kits. This is screwed in to the number 1 spark plug hole and you attach a hose to the other
side. Crank the engine until air begins to rush out the hose (put a balloon on the end of the hose if you want
to get some visual feedback, I've never tried this but it seems like an interesting approach). When the air starts
coming out, it means you are headed into the compression stroke. Then slowly continue turning the engine until
the number 1 piston reaches its highest point. I have determined this by using a very long thin plastic rod to
feel how high the piston is.
Some caution is advised. I mentioned a long rod because a short one could be dropped into the cylinder forcing
you to remove the head to retrieve it. Also be careful not to disturb the carbon on the top of the piston since
it could get stuck in an exhaust valve when you finally start the engine.
Bolting on the Magneto
To get your magneto to mesh to the engine, you must turn the magneto (in the direction it was designed to operate
in) until the rotor is positioned over the number 1 plug wire terminal. When positioned thusly, the dogs or slots
on the mag should line up with the corresponding dogs or slots on the engine and slide right in. Loosely bolt on
the magneto such that it can still be turned but is held in place. Connect the grounding wire. Be sure your grounding
/ kill switch is in the off position. You may not have a ground wire if your magneto is one that requires you get
off the tractor and ground it manually to kill the engine. In this case, it is a good idea to make a wire to ground
it temporarily or the engine may start or seriously backfire unexpectedly during the final timing.
The plug wires should be connected in the order specified in your manual (Note: if you are not using what your
manual shows as number 1, hook the number 1 wire to the cap terminal that you timed to be number 1 since this is
all that really matters). The firing order and direction of magneto rotation direction are the keys to hooking
up the rest of the wires. Examples are the Allis WD, WC, WF, B, C, CA, and IB use 1-2-4-3, the Farmall Cub, Allis
G and MH Pony use 1-3-4-2. If no manual is available for your tractor, you can determine this by watching the order
in which the intake valves open (rocker arm will go down when the valve opens). Determining the firing order this
way involves removal of the valve cover.
By this stage, you have a rough timing but don't try to start it this way. Make sure the magneto is grounded
(turned off or a "crowbar" safety wire hooked up between the ground lug and a good ground on the tractor)
or you might just break your arm or worse. In this state the tractor can backfire and run pretty ragged. If you
are using a hand-crank, it can furiously spin backward and kill or seriously injure you.
To get that final timing, rotate the body of the magneto counterclockwise (Note: whether counterclockwise or
clockwise will depend on the rotational direction your magneto is designed for, the rotation you want here is the
opposite of the normal rotation, counterclockwise works for Allis and Farmall letter series machines from 1938
till magneto's disappeared). Slowly turn the engine through its strokes till you have the number 1 piston at the
top of its compression stroke again. Now gently rotate the magneto clockwise until you hear the pronounced click
of the impulse coupler. This indicates that the magneto is right at the point where it will fire the number one
piston. Tighten up the bolts, remove your temporary safety ground wire (if you put one on) and try starting the
engine. This is the point where, if you are hand-cranking the engine, you must follow the hand-cranking safety
precautions in your manual since you are trying to start an untested engine. Since there are several factors beyond
mag-timing that to go into backfires and you may have made a mistake making this the most dangerous time for hand-cranking.
After this process is complete, it is up to you to figure out if any further minor adjustments will help your
tractor when running at speed. I have done minor adjustments at this point when the engine is running to improve
running at rated RPM. In general, your timing should be very close at this point.
You may have timed your magneto correctly but the tractor still won't start. The many things that can go wrong
are for the most part beyond the scope of this article but a few that are related follow.
We mentioned an audible click that should occur when the engine comes to the top of any compression stroke.
This is the impulse coupler and is what allows a tractor to start at cranking speed. It fires an extra hot spark.
If you do not hear that click, your magneto may have a broken impulse coupler. It is possible for a tractor to
start with a dysfunctional Impulse Coupler, but to do so requires a perfect starter that can crank the engine at
fairly high RPM. Your options for fixing this are to buy a new magneto, send your magneto to one of the businesses
that restore them or buy the "How to Restore Tractor Magnetos" Motorbooks reference by Yerigan and do
Confusion on Number 1?
Some caps have a 1 at the contact where, by convention, the number 1 spark plug wire should plug in. In some
cases, this conflicts with what the operators manual may indicate is the number 1 plug contact. In this case, if
one were to time the magneto according to the manual but hook up the wires according to the number on the cap,
the machine might still not be timed. What's important is for you to keep track of which contact you are timing
the rotor to inside the mag and hook up the number 1 wire to that contact on the cap.
Before you spend too many days cranking or wearing out the charge on your battery, check the spark. If the magneto
is functioning, you should be able to get a spark. Most auto parts stores carry a spark tester that makes it pretty
easy. It looks like a spark plug with a large alligator clip. Hook this "Spark Plug" to a plug wire and
connect the clip to ground. When cranked, it will flash if you have sufficient spark. If you can't see a spark,
you are wasting a lot of energy cranking.
If you aren't getting spark, disconnect your ground wire to the magneto and try again. Remove all the plug wires
from the plugs to keep the engine from starting since you are testing for spark, not trying to start it. The kill
switch could be shot and permanently grounding the magneto. Pulling the ground wire off is the same thing as disabling
the kill switch or more directly "turning the key on". If the tractor does start, ground the magneto
immediately with a jumper and kill the tractor. This is critical since, depending on the wiring of your electrical
system, you may be running with it disabled (since the ignition switch is the component in question). This can
damage the generator.
Where to find the specifics for your tractor
Most service manuals do not cover the process of magneto or ignition timing. Since the service manuals were
designed for the dealer who had trained mechanics, they must have assumed that this process was as intuitive to
the dealer mechanic as putting air in the tires. As a result, you nearly always find the Magneto timing in the
Operators manual since this is what every tractor owner would receive with the tractor. Usually both manuals will
have the firing order.
A word of caution on following the factory manuals. Your tractor may have a different magneto than the normal
magneto supplied originally. An example of this is that the Allis Chalmers B manual shows a Fairbank Morse Model
J while many ended up with WICO magnetos as replacements. Aligning the internal gears is different on the WICO,
thus the manual can't help in this respect.
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