As engine collectors grow older, making low-tension, rotary magneto, igniter-fired engines easier to start becomes a high priority. Cranking speed is likely down in the 50 RPM range, where the magneto output is well below that when running at the rated RPM. Easy starting is very subjective. Easy to start for one person may be hard to start for another. That said, increasing spark energy at low RPM makes an incredible improvement in ease of starting.
In street vernacular, the claim is that a hotter spark is the answer to an easy-starting engine. It should be pointed out that the actual temperature of a spark doesn’t change with spark strength. In physical terms, a hotter spark is one with more energy. Over the years, there have been several articles in Gas Engine Magazine related to spark energy and, therefore, ease of starting. “Getting an Easier Start on a Rotary Magneto Igniter Engine” describes replacing the OEM magnet with a rare-earth-enhanced magnet. The willy-nilly placement of rare-earth magnets on a magneto will yield disappointing results. If, however, the rules outlined in that article are followed, a six-times improvement in spark energy can easily be achieved. A second method, outlined in “How to Bench Test a Magneto,” rewinds the armature with modern insulated wire. If the engine is Model T buzz coil fired, “Gas Engine Buzz Coil How-To” describes a buzz coil made with a modern high-energy automotive coil. Any of those methods will greatly improve the starting of a well-tuned engine.
Another method is to start the engine on a battery and coil. When the engine is up and running, unhook the battery and coil and reconnect the magneto. This method has advantages and disadvantages. There is almost no limit in achievable spark energy by using a high-voltage battery and low-resistance coil. This, however, comes with the risk of burning out the coil due to excessive heat. The term “hot box” is used when the battery and coil are put in a box with external connectors (see Figure 1). The connection and disconnection as well as storage of the hot box every time the engine is started soon becomes a burden. When in use, after the magneto has been disconnected, the red wire in Figure 1 would connect to the igniter and the black wire to the engine block (ground). The red and black wires can be reversed depending only on your desire to have the electrons jump left-to-right or right-to-left across the igniter.
This article describes a hot box that is small enough to sit on an engine skid. This design also has a toggle switch to move back and forth between magneto and battery and coil with ease. A third position of the switch shuts the engine off. It was purposely designed to be small and set on an engine skid while delivering the spark to start an engine 15 or 20 times at a day-long show. It was not designed to provide hundreds of starts or to power a belligerent engine while troubleshooting other problems. The hot box is fully rechargeable using any 12V charger or battery tender. Everything needed to build this switchable hot box is shown in Figure 2.
Automotive ignition coils (1940s through 1970s) that are readily available, inexpensive, and rugged make nice coils for battery-and-coil systems on antique hit-and-miss engines. For this project, only the primary winding of the car coil is used. The coil described here is a UTC12T found on Amazon for about $15. However, most any coil can be used. When a car engine is started, the coil is in continuous use and dissipates a lot of heat. Therefore, automotive coils are kept in an oil-filled can to aid in cooling and have a large, high-voltage tower, both of which take up a lot of space. The coil can be made physically smaller because this hot box won’t be in continuous use and won’t use the high-voltage coil. Figure 3 is the original coil.
In Figure 4, the can has been cut open, the oil drained, and the workings pulled out. There will be a metal core made up of several thin sheets of metal stood vertically in the can. Around that will be the secondary coil. The secondary or high voltage coil will be many turns of a very fine wire. One terminal of the secondary coil will be attached to a compressor fitted to the plastic HV tower. Around the secondary coil will be the primary coil. The primary will be a few turns of a much larger wire.
Snipping the primary coil terminal wires from the + and – terminals on the HV tower results in a nice, compact coil for the switchable hot box. The final coil, as used for this project, is shown in Figure 5. By leaving the core and secondary coil in the assembly, you’ll obtain a slightly more energetic spark when compared to removing them. If space isn’t important, the coil can be used without disassembly.
The battery, shown in Figure 6, is a CA1208, 12V, 0.8AH and was picked for its small size, keeping in mind that the final hot box will fit neatly on the engine skid. That battery can be found on Amazon for less than $20. At only 0.8AH, the number of starts will be limited. John Deere, for example, keeps the igniter closed full time except for the brief period of the igniter trip. Engines like the John Deere will likely get 15 starts before recharging is necessary. Engines that leave the igniter open until shortly before the igniter trip will likely get 50 starts before recharging is necessary.
The switch is a single-pole (only one switch), double-throw (will connect the middle terminal to either of two other terminals), three-position (center position is off) switch (SPDT 3 position). On Amazon, these are often called “ON OFF ON switches” and can be found for under $10 with a 5-to-10A rating. Figure 7 is a conceptual drawing of the operation of a SPDT three-position switch.
With the handle in the up position, terminals 2 and 3 will be connected (shorted) internally. With the handle in the middle position, no terminals will be internally connected. With the switch handle in the down position, terminals 1 and 2 will be connected together internally.
The name of the game in a battery and coil setup is current, not voltage. The spark energy goes as current squared, I². Therefore, spark energy goes up rapidly with increasing current. Twice the current results in a four-times increase in spark energy. For that reason, most antique coils as well as the automotive primary coil used here are very low resistance. A typical primary coil of 1.5 Ohm resistance connected to a 12V battery will conduct 8 Amp while the igniter is closed. Wires of 20 gauge or larger should be used. Remember, electrons are colorblind, so the color of the wire
is unimportant.
Going back to Figure 7, the igniter will be connected to terminal 2 of the switch. Terminal 3 of the switch will be connected to the magneto. As in Figure 7, if the switch handle is up, the magneto will be connected to the igniter. The battery and coil network will be wired similar to Figure 1 and connected to terminal 1 of the switch. The final schematic is shown in Figure 8.
A convenient way to identify the terminals on the outside of the completed box, is to paint the knurled thumb nuts. In this build, the terminal going to the engine ground or block will be black while a red thumb nut will indicate the magneto connection. The igniter connection will remain brass.
For those not familiar with electrical schematics, Figure 9 uses jumper wires to show how all the components will be connected inside the box. In Figure 9, the black wire from the battery (-) will go to the black thumb nut terminal on the box (ground). The red wire from the battery (+) will attach to one terminal of the coil. The yellow wire attaches to the other coil terminal and goes to
terminal 1 of the switch. The green wire goes from terminal 2 of the switch to the brass thumb nut (igniter) on the box. The white wire goes from terminal 3 of the switch to the red thumb nut (magneto) on the box.
Figure 10 shows all the components tucked into the box and connected.
Figure 11 has the box closed showing the brass thumb nut that will go to the igniter, the black thumb nut that will go to the engine block and the switch. The magneto thumb nut, red, was placed on the rear of the box facing the magneto.
Figure 12 is the switchable hot box connected and sitting on a John Deere 3hp skid.
To recharge the hot box, connect the positive terminal of the charger or battery tender to the igniter (red) terminal and the negative terminal of the charger to the ground (black) terminal and put the switch in the battery and coil position. Because battery tenders have a peculiar method of charging and measuring, the coil in series with the battery sometimes confuses them. Left alone, however, they’ll charge the battery fully.
Dr. David Cave is a regular contributor to Gas Engine Magazine and Farm Collector. He can be reached at jdengines@cox.net.
