russ Robertson

There are options when selecting a replacement ignition coil (Magnetron) for the mower engine. I was interested in changing over to positive high voltage rather than normal negative high voltage. There are a couple reasons. When tinkering with small engine for alternate uses/ fuels, There are high voltage shocks received when connecting parts such as resistor, capacitor, L coil, to engine ground. The spark plug refuses to fire, and bypasses to go up another line such as parts. During that time we get shocks from the engine. Also, when using the original old style Magneto as just a plain transformer before 1982, you can put it upside down on engine bracket and get positive high volts, but at away reduced level. You don't get up to 15kv anymore. The Autotransformers normally have the primary coil hooked directly with secondary side. The full secondary is normally primary + secondary in a negative "mode" Less wire is used in the positive mode.

Now then, there actually is a better way to get positive type high voltage at FULL VALUE VOLTS! If your Briggs engine is a one cylinder type, you can purchase a modern replacement Magnetron with DOUBLE CABLES. The secondary side is not connected with primary. When used on the twin cylinder engines, each spark plug gets different type power. Negative on one side and positive on other cylinder. Also, if you get the most modern type dual cables Magnetron, it will have the better type built in UNIVERSAL type trigger , rather than the Briggs brand trigger. Engine will fire at a lower RPM of flywheel , just like in the "olden days". Just hook the negative cable to ground and the positive cable to spark plug. Compare the photos of armature Magnetrons on Ebay to see the difference.

It's my current assumption that the conversion circuit of S1R9A9M9 2008 lawn mower water engine 18HP on You tube videos, had used a hidden "small dimension size" Inverter box, about 300-500watts , that was INSIDE the rear hole of cement block with engine on top . Plenty of clearance also for the vertical rotating crankshaft. The other parts were hidden in sealed, painted plastic tub , hanging on side of engine. we could see. Therefore, the Inverter power source was hidden also, so as after the battery was completely disconnected, the engine conversion circuit ran off of the alternator installed under the flywheel. The Briggs stator of yellow+ yellow wires is capable of 20amps dc current. The ignition only operates during about 34% of flywheel turning, so as the wattage from Inverter box is reduced by 34% also. So as current delivered in DC pulses was 6-7-10 amps DC., The hidden Inverter box was being operated in the upper region called "SURGE current" which these boxes are capable of. The pure sine wave type output  boxes give much higher surge currents, than the modified sine wave type which is square wave pulses waveform  on an oscilloscope. Remember the significance of the very low duty cycle. So now you know. The 400watt green Inverter shown in videos,was not used-, not hooked up,  not because it couldn't run the engine, but because it was too big to hide in cement block. The conversion circuit required about 100 volts and current same time. The other S1R9A9M9 electromagnet provided the magnetic field to change the timing to extend to negative late timing, and also provide required field for the tips of spark plug. The high voltage cable hooked to the heavy current same time as going to the zero ohms spark plug. Hydrogen gas evolved and fired along with generation of water vapor pressure.
7amps X 110v  = 770watts  times .34 duty cycle flywheel usage, = 260 watts or less drawn power , plus the electromagnet current . Both draws are capable of the on board changed alternator. The black+ black wires Briggs alternator is the exact same low ohms as the yellow yellow. Just get the cheapest one. The 500W pure sine wave Inverter is about $40 called LVYUAN painted blue color. from US EBay shipping.    Outer size 3.75" wide by 7.8" depth - IT FITS very loose in cement block, or 300W red LVYUAN 3 1/4" by 3 1/4" size.