The SF7 and VMN magnetos used on the W-670 do not have the “Impulse” rotational feature that gives quick magneto action, even at low cranking speeds. When the W-670 turns at slow cranking speeds on start, so do the magnetos. And unfortunately, the energy in the spark at the plug is very dependent on the magneto turning speed.
One way around this problem, is the installation of a Booster coil. The booster coil is powered by 12 or 24 volts, and uses a vibrating reed to put out a pulsing high voltage onto a high tension lead that attaches to one of the mags. These mags are designed to accept a lead from a starting coil.
The SF7 mag has a special port just for that lead on the front top of the mag. On the VMN mag, the lead goes in the port on the elbow and follows the rest of the bundle down to a special attachment on the mag block.
This lead then puts a continuous shower of high voltage, high energy, and sparks onto the “retarded finger” in the magneto, giving a good hot spark for starting even at low cranking speeds.
Typically, the booster coil is energized in parallel with the engage solenoid on the inertia starter, or just in parallel with the direct drive starter like the E-80. And it makes a world of difference in starting reliability. Most of the Bendix booster coils are labeled 24 volts, but the tension of the vibrating reed can be adjusted for 12 volts and works just fine. They are labeled as being adjusted for 1.8 amps DC at 18 volts.
Another point in favor of the booster coil is that the original configuration of the mags included a mechanism for rotating the points of the mag to a retarded position during start. This involved a handle near the primer that the line chief pulled to engage the inertia starter. This handle also rotated the point plates on both of the mags to a retarded position while the starter was turning the engine. This position enabled easier starting. A spring returned the mags to the advanced position when the start handle was released.
A secondary disadvantage of this configuration is that the rod connecting the two mags together for the retarding action, is smack in the way of removing the oil filter screen from it’s housing during inspection at oil change, and requires removing the rod with it’s small clevis pins and cotter pins (and hunting for them in the bottom of the engine compartment).
However, with the installation of an electric motor for inertia wheel run-up, with a remote engage in the cockpit, and direct drive starters, the “retard during start” mechanism gets complicated and is often abandoned. Some Stearmans had a manual retard that partially solved this problem, but required another hand. So the magnetos are left in the normal advanced position during start, and it works pretty well, but not as well as having retarded timing. In this event, the coil plates are locked down in the advanced position.
The booster coil solves all of these problems and also gives a hot retarded spark at slow cranking speeds.
For the booster coil high tension lead, it is better to use shielded ignition wire or flex conduit from the firewall to the mag. Even though that lead is active only during starting, it could act as a passive antenna and contribute to ignition noise in the radio.
There are some VMN mags that have an automatic advance feature that allows the mag to start in a retarded position and advance the spark as the engine comes up to speed. These mags were not originally approved for the Stearman, but I think that they are now covered by an STC.
Since these mags are retarded when not turning, in order to set the timing one must subtract the degrees of advance from the 29/32 BTDC numbers and put the mags at a new retarded setting. The idea is that when the mags do the automatic advance, they will arrive at the 29/32 settings for normal running. But there is no way to check them at those points.
The Bendix Booster Coils, type C-1, are available at Dusters and Sprayers, Air Repair, and Banaire Enterprises in Hemet, CA, 909-927-9714. The Booster coil is 3 3/8 inches wide on the mounting surface, 3 3/8 inches long, and 2 1/4 inches high from the mounting surface.