What's a Magneto?
On an old-fashioned auto engine, you have a combination of a coil and a distributor to create a spark and get it to the spark plugs. A magneto does that in one unit. The car also has a battery to provide the electricity to the coil and a generator to keep the battery charged. The magneto makes the electicity, too. It can do other tricks, too. We'll get to that.
How does it work?
The most simple form of magneto is in your lawn mower. A magnet in the flywheel spins past a coil which creates an electrical charge. Points (a switch) open at just the right time causing the charge to be stepped up and released to the plug. A condenser (a capacitor in anything other than an engine) controls the discharge and keeps the points from burning up quickly. In a four-cylinder magneto, a cam with 4 bumps or lobes causes the points to open 4 times on one turn, making 4 sparks. A rotor on the top of the shaft lines up the spark to the appropriate connection on the distributor cap so the correct cylinder fires.
You probably know from experience that you have to give that rope a good pull to start the lawn mower engine. The faster the magnet and coil pass each other, the hotter the spark. You can probably imagine that spinning up a tractor engine is a little tougher and it is. Therefore, a handy thing called an impulse coupling gives a hot spark with just the slightest motion. As you rotate the crank, a spring tension builds in the coupling instead of rotating the shaft. When the piston passes Top Dead Center (TDC), the coupling releases, creating a serious spark. Some guys claim they have to leave the crank handle down, lest a bird land on the crank and start the engine! Once the engine is running, centrifugal force shuts the impulse down and lets it run at shaft speed.
If you have the mag off of the engine, you will find that you can turn it freely in one direction, but not the other. If you can turn it in both directions, you either do not have an impulse coupling or you have one that is broken. (Airplane engines have 2 magnetos. The left one has an impulse and the right one does not!) DO NOT fire the magneto unless you have the coil shorted or connected to a spark plug that is grounded to the mag. That spark has to go somewhere and it can break down the insulation, ruining the coil.
Why a magneto?
The earliest models did not have batteries and electric start. The same is true of WWII models. Therefore a magneto was a necessity. Whether or not you have a battery, a good magneto makes for a great-running engine.
How do you work on one?
Normal maintenance is about the same as with a coil and distributor ignition. A tuneup will require points, condenser and cap. The points have to be set right to get a good spark. The timing must be set right. If you can tune up a 1964 GTO you can tune up a tractor with a magneto.
On our Case, we find it easiest to work on it off the engine. We tried leaving it on, but ended up dropping things into the grass! Usually just 2 bolts and some wires attach the thing to the tractor. That part is easy. Some magnetos have a bar on the shaft that fits into a slot on the tractor (or vice versa). These are great to work on. Just mark the mag and block before you loosen the bolts. If you do not turn the mag or the engine, you can just slip it back in and bolt it down when you are through tinkering. Others have two gears that have to line up just so. These require a little more care. We'll get to making sure it is right in a bit.
Our big Case had been sitting up for a number of years. Someone had started a restoration and then left it out in the yard for years. Everything turned and it looked like the major tune up components had been changed since it last ran. It would not make a spark. We opened up the cap and got all of the ants and spiders out. That's always a good place to start. Using a meter, we discovered that the points were never making contact. More accurately, they were closing, but not making electrical contact. Corrosion on the contacts had formed an insulating layer. Somewhere in the process, we gave up and pulled the mag off the engine. Before doing that, we pulled the plugs and used a screwdriver to find number 1 cylinder come up to TDC on compression, just past where the mag fired. We also marked with masking tape labels which wire went where.
With easier access, we were able to easily sand the points. With the cap off we could not connect plugs to test fire, so we held a screwdriver against the case and near the coil button. Turning against the impulse, we were able to get no spark, but holding our hand across the connecton we could get a fair tingle. We were on the right track. Adjusting the points, were were then able to get a nice spark. If this does not work on a mag, you might need a new coil. Most are still available. Check your dealer or a NAPA store for local sources.
By observing where the mag was going to fire next, we got it back to where it was when we took it off and bolted it back on. Easing the crank around, we observed that the impulse snapped just past TDC on the next cylinder. If you fire just past TDC the engine will start easily with no kickback. If you fire prior to TDC, you could break an arm or get launched into the next county! If your mag has a gear, you may have to do a little trial and error to get the teeth lined up right. You may find the timer about to be described to be useful, too.
Once you have an engine running it is a simple matter to use a timing light to set the timing by loosening the mag bolts and swinging it one way or the other. There is a neat way to statically time the thing without the engine running, using a device generally unknown to tractor mechanics. Airplane mechanics have a device called a mag timer. It is actually a dual timer since planes have two mags which have to be set exactly the same. One clip goes to ground and the other goes to the kill lead of the mag. The third clip won't be used on a tractor. A tone changes when the points open and close. Turn the engine backwards (CCW from the front) to avoid the impulse and get a true reading. Without the impulse, you will note that the points will open before TDC (or after since we are turning backwards). This is a good thing. These mag timers are available at aircraft tool suppliers and cost about the same as a high-end timing light.
If you have a mag that has been left out in the weather, you might consider trading it in for an overhauled unit or sending it to a shop. If you are the adventurous type, use the usual application of your favorite penetrating formula and some gentle tapping and prying to loosen things up. If, after you loosen it up, it does not feel too loose and wobbly, replace the tuneup parts as needed and see if you can make it spark. It things are loose, you need to look into some new bearings or bushings. Wobble can cause erratic firing, including the kind that will break your arm trying to start the thing!
While you are working on the ignition, treat your tractor to new plugs and spark wires. Plugs are usually only a dollar or two each and a four-cylinder wire set is about $9-15. It will look better and that 20 bucks spent might save you a lot of troubleshooting time.
We'll be adding illustrations soon!