Getting an Easier Start on a Rotary Magneto Igniter Engine

article image
by Dr. David Cave
Figure 1: Bare Bones Charger charging a horseshoe magnet.

As engine collectors get older, the need for an easier starting engine becomes a higher priority. Following are five DIY ways to improve the starting process. All five will work for any rotary magneto igniter-fired engine.

These ideas assume the engine is running and in sound mechanical condition to include adequate compression, proper timing, properly aligned mixer needle valve seat, no mixer air leaks, functioning valves (seats, guides, etc.), check valve is working and the igniter is not worn or gummed up. These five ideas are in no particular order. A solution for one situation might be the least desirable in another.

With the proper equipment it is easy to measure spark energy or its increase. However, ease of starting is subjective and difficult to quantify. One person’s easy starting engine may be difficult to another. Some armatures on the market improve spark energy 1.8-times. It is difficult to say how much this will improve the ease of starting. When using any of the described techniques, your milage may vary.

1. Recharge the magneto magnet

The basic laws of physics show strong magnets create strong sparks, but these magnets use 1900’s technology and slowly degrade over long periods of time. Most engine clubs around the country will have a member with a charger who will recharge the magnet for little or no cost. Closed engines, like the John Deere (see Figure 3), would seem to make this rather trivial task more difficult because at least two oil seals must be opened to get the magneto off.  However, it is not necessary to break the engine down (crank cover and governor box plate) to remove the magneto. Multiple studies have shown the oft quoted “never remove a magnet from a magneto” to be false. Pull the magnet off and charge it. If no charger can be found, you can learn how to build a magnet charger for about $40.

2. Replace the magnet

This is a bit tricky. There are many rare-earth magnets for sale on the internet that are no better than the original John Deere magnet at cranking speed. Statements like “It’s hot” or “It makes your engine easy to start” or “The strongest magnet on the market” are not always indicators the magnet being advertised will produce a hotter spark. Ask for real data (spark energy) or ensure the magnet was manufactured following the rules in our Evac magnet plans, which has instructions on how to build a strong magnet.

A well-designed rare-earth magnet will increase the ease of starting a mechanically sound engine more than any other change. Simply remove the magnet strap, pull the original magnet off, and push the new one on. The new spark energy is not free; the magneto will be harder to turn — possibly increasing gear wear. Unless you intend to run your engine for several thousand hours, the increased gear wear will not be an issue.

3. Connect a battery and coil

It is well known that a low-tension rotary magneto engine will start much easier when the magneto is replaced with a battery and coil. Although this approach can be simple (auto coil and a 12v battery) it requires taking those extra components to the show and switching back to the magneto when the engine starts. The author and others sell a small hot box with a 3-position toggle switch, see Figure 3, that can be attached directly to the engine skid. With the switch in the down position, the engine can be started on the box’s internal battery and coil. Once running, the switch is then moved to the up position to run on the magneto. The switch center position shuts the engine off.

4. Advance the igniter timing

A search of John Deere Model E literature will reveal two conflicting timing specs. One spec states the SPARK mark on the flywheel should line up level with the push rod when the igniter trips, see Figure 4. This is by far the most common method used to time John Deere Model Es. It is not known if John Deere was optimizing power, fuel economy, or some other feature. It does not optimize spark energy. By the time the igniter trips, the magneto has passed its peak output.

A different method of timing a Model E can be found in John Deere magneto’s literature. On the backside of the Iowa Dairy magneto are L and R push pins, see Figure 5. The magneto peak output occurs when the L (L for a John Deere, R for an Associated 2-bolt) push pin drops into its detent. By having the igniter trip when the L-pin drops into its detent, the spark is advanced about 5 degrees and a 1.9-times hotter spark is delivered. In Figure 6 the flywheel is at the position when the igniter trips if timed when the L-pin detents.

5. Rewind the magneto armature.

Modern wire and insulation technology has advanced well beyond what was available in the 1920s. Modern high voltage insulation is much smaller in diameter allowing for a much higher packing density and eliminating the need for spacers. More turns of wire can be packed in the limited armature space resulting in the armature coil having a much higher inductance. Rewinding your own is likely not an option, but a source for rewound armatures can be found on Facebook at The U.S.A. John Deere Engine Group. Those armatures, Figure 7, produce a spark, at cranking speed, that has 1.8-times more energy than a standard John Deere armature using the same magnet. The improved starting ease for $125 is a pretty good bang for your buck. The down side is that removing and replacing the armature requires opening up the engine crankcase.

Dr. David Cave is a regular contributor to Gas Engine Magazine and can be emailed at

Gas Engine Magazine
Gas Engine Magazine
Preserving the History of Internal Combustion Engines