The electrical circuit for the water powered mower engine has several voltages involved that you have to be aware of.
(1) fully charged battery voltage
(2) Inverter box secondary AC voltage
(3) AC voltage reducer box voltage output
(4) Full wave rectification DC block output power voltage
(5) Voltage drop loss adjustable power resistor
(6) Voltage drop loss 15KV series diode bank
(7) electrical load spark plug conductive electrolysis cell voltage across
(
Ground return line 1 ohm resistor voltage drop back to Inverter ground terminal
(9) In series "added" voltage from cross leakage induction volts from S1R triple coils assembly
(10) Briggs alternator ring output AC voltage
(11) Briggs alternator voltage regulator output DC charging voltage
(12) voltage drop loss from accessory in series spark gap with spark plug
(13) voltage drop loss from inductor 1.5-1.75 mh in power line
E/I=R, E/R=I , I X R =E Voltage drops in series add up to supply volts.
An electrical meter as multiple neon bulbs ,or digital readout voltmeter will only register the highest voltage at top of the spark plug. What's there, is about 35vDC, 100v added DC, and variable high ignition volts DC. What is also needed, is to measure separately and KNOW the crossover leakage volts that ALLOWS the spark plug cell to make sufficient hydrogen gas. This voltage is adjustable by using the common spark tester with screw that adjusts ,as used by auto mechanics This is the accessory spark gap.and is in line with the 2nd inductor, and spark plug to ground. ( With the plug gap covered in conductive electrolyte such as baking soda water, salt, the plug gap greatly reduces the high voltage. The 1st spark gap can regulate the final gap, and also the needed induction crossover volts that pushes faster hydrogen gas.The S1R9A9M9 coils has the 2 red wire ends that receive the induced voltage. The GAP adjustment is done to get this induced voltage sufficiently high. Remember, the diodes bank must cover the highest volts ,so as no return damage to inverter or battery. The Briggs engine is called a 4 cycle and has what's called WASTE SPARK. During that spark period time of flywheel rotation, of no moisture plug gap,and is an open air gap, the ignition voltage jumps up way higher. Now you see this is a more involved design for a bench set up. Temporarily, I use a small old style Harley motorcycle ignition coil can with push/release points switch and condenser for immediate testing with high voltage.
On the old cars with high mileage, after dark, you lift the hood and see sparking going on with the ignition wires. They are worn out from overheating and needed replacing. The engine was running uneven because of crossover induction. The 1978Waterfuel web forum, that's now shut down, had page reference to car ignition thick insulation cable wire wrapped with 60 turns insulated hook up wire for purposely getting crossover leakage volts!