Hardly anything is more frustrating than getting ready to go flying on a beautiful day and then finding that your engine will not start (unless it is flying the first leg of your trip only to find that your hot engine will not restart!). Lycoming-powered Stearmans and Continental-powered Stearmans share some similar starting problems, yet have some characteristics unique to their individual power plants.
The R680 Lycoming engine, whether in the 225 HP or the 300 HP configuration, utilizes the DFN dual magneto (two magnetos driven by the same shaft and housed in one case). Originally a fairly trouble free unit, the DFN is plagued today by the same problem facing most WWII vintage electrical systems. Coils and condensers built in 1943 were not designed to sit on a shelf for fifty years before use. We often find that in a stock can of vacuum packed 10-7713 condensers, three or four of the eleven in the package are non-functional straight out-of- the-can! The other seven or eight may or may not last to TBO. Coils, likewise, absorb water, which causes internal corrosion and a resultant breakdown under load. In our shop it is standard practice to test run the magnetos with a heat lamp simulating real conditions in the air, thereby weeding out the new defective components.
Another complication unique to the R680 powered Stearman is that most operators are now equipped with shielded ignition systems, the most popular of which is the stock R680-13 system. This assembly utilizes a front mounted loom behind the push rod tubes with two 1-1/2” braided brass conduits running back between the cylinders to cast aluminum shields covering the distributor heads (caps). The problem is that the cast aluminum shields (which were never originally installed on the Stearman) interfere with the engine mount. We often see caps that have been ground away in an attempt to clear the mount. The end result is that it is almost impossible to remove the distributor cap to inspect the inside of the distributor. Oil seeps past the distributor seal, the carbon brush in the cap wears away, and the electrode on the distributor finger erodes, all of which cannot be detected without removing the distributor cap and all of which contribute to hard starting.
A common “remedy” to all of this is to add a shower-of-sparks booster coil. This coil feeds into the right distributor and provides a retarded, very hot spark for starting. However, using a booster coil to overcome oil seepage and wear in the distributors is like the man whose ailing dog’s nose was hot and dry. Now this guy knew that a healthy dog has a cold and wet nose, so he held ice on the dog’s nose until it was cold and wet. The dog was not any healthier, however, nor will your Stearman be if the booster coil is only being used to mask other problems. The only real fix is an inspection and repair of the distributors, however difficult that task becomes with a shielded system.
Much of what was said about Lycoming magneto condensers and coils applies to the SF7RN-l and VMN7DF magnetos found on the W670 engine as well. They were just not designed for a fifty year shelf life. Another factor with the W670 mags that contributes to hard starting is weak rotating magnets. When originally built the rotating magnet within the magneto was charged on a magnet charger (making it a four pole permanent magnet). Unfortunately, the magnet can lose some of its magnetism over time, thereby producing a weaker spark and making the engine harder to start. The remedy is simply to have the magnet recharged.
Recently we have seen a rash of VMN7DF and VMN7DFA magnetos failing due to lack of lubrication. The early symptom of this failure often is a hard to start engine. Originally oil passages were provided in the magneto base for the front ball bearing and the plain bushing on the large distributor gear, and in the coil cover for the rear ball bearing. In service it was found that in order to sufficiently oil the distributor gear bushing, the ball bearings were being over oiled. Bendix issued Service Bulletin #327 in February of 1955 instructing that the oil passages to the ball bearing were to be sealed off and that the ball bearings were to be packed with magneto grease. The oil cup on the coil cover was to be soldered closed and the oil cup on the magneto base was modified to oil only the distributor large gear bushing. Since this passage continued past the bushing through the back of the magneto, it became impossible to over- oil the magneto. Excess oil simply drains out the back of the mag. Bendix does not give an interval for oiling the magneto, but in our experience, 25 hours is about as long as it should go without re-oiling. Without oil the distributor gear becomes very hot and the magneto literally melts down. Remember, it is impossible to over-oil the magneto (if modified), but very easy to under oil it.
Though worn spark plugs, defective ignition wires, leaking cylinders, slow cranking starters, and carburetor problems can all contribute to difficulties in starting, most of the Stearman hard starting problems can be traced to magnetos and distributors. Well maintained magnetos and distributors are the best defense against a hard to start engine.