I ran into a problem Monday, time for the first lawn cutting of the year, trust me, at 73 I don't even use a riding mower because the vibration would kill my artificial knees, our landlords are a set of identical twins who love yardwork (as far as I'm concerned you could cover the front and back yards out to 50 feet from our home with a foot of small gravel and spray with an herbicide at regular intervals.)

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I did all the normal checks. The fuel was drained at the end of the season, from the fuel tank and carburetor bowl, the engine ran with fogging oil and the air filter cleaned, then the riding mower was placed in the detached garage.

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For the new season, the tires were checked and topped off, a new spark plug was installed, good gas used to fill the tank.

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The battery was on a float charger all winter.

Turning the key to start caused the starter to crank with normal speed but no joy, not even a backfire. I used some ether (I'm lazy) and got a feeble backfire.

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Testing with a spark plug showed an arc the gap, but, at peak compression it takes a lot more voltage for the arc to form, how much more voltage, I'm not sure. But a mechanic told me "Up to 4 times."

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OK how on earth do I measure up to 20kV?

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The Leslie Corporation offers a nifty inline tester with a neon bulb with terminals on each end. The tester goes in series between the ignition coil output and spark plug hot terminal. If good, as in adequate, high voltage is present, the neon bulb will flash pretty brilliantly, if the high voltage is low, there might be a dim glow. The Leslie

20610 INLINE SPARK TESTER is available at Advance Auto or online. I went with Advance Auto because we decided time was more valuable then a few dollars.

It was money well spent.

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Changed the ignition coil, PITA to get to, and presto, our landlords think I'm a genius. [Ha do I have them fooled or what?]

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We also used compressed air to blow debris out of the ventilation fins on the flywheel and generally clean the engine.

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The engine started instantly and ran perfectly. Looking back it is clear the ignition coil was dying over the last 2 or 3 summers because the engine runs smoother with a LOT more power.

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Our next joyful task will be to change the drive belts. They are a little frayed and I feel it's a lot better to change them when it's 50 then 90 with 80% RH. [Yippee I can't wait.]

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On Mar 21, 2025, at 3:54 PM, wn4isx via groups.io <wn4isx@...> wrote:

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Somehow the idea of using a HV probe that requires Freon cooling is a bit beyond the pale even for me!

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My Bird Termaline Watt Meter is rated at 60W, I mounted 2 4" 120V muffin fans against the heat sink and I've ran 150W into and it didn't die, didn't even get hot enough to smell oil.

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But a HV probe that requires Freon cooling gives me the willies.

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And I bet the Leslie probe cost a whole lot less then the Tektronix probe, even if it doesn't display the waveform out to 100MHz or so.

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Sir you've worked with some very exotic equipment!

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A potential problem is the voltage also has to have enough current when the cylinder is at the top of the compression cycle and a voltage reading under "no load" might not indicate the voltage when the spark plug is under maximum compression.

The Leslie unit actually indicates voltage/current flow through the spark plug via the arc.

Proper high voltage will make the neon bulb light up rather brilliantly, weak high voltage/current will give a very weak light output.

I considered the style indicator you suggested but a mechanic told me to spend a bit more for an indicator that actually shows you what is going on.

The Leslie costs about $20 and worked very well for our needs.

Most modern vehicle ignition systems have "coil packs" per spark plug or for two spark plugs.

The later doesn't ground the one end of the high voltage winding, it is double ended and feeds one output to one spark plug and the other output to the adjacent plug. This was more common in the 1995~2010 time frame or with inexpensive [cheap] cars today.

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My wife's 1998 Ford Escort ZX2 used this scheme and called the spark in the non firing cylinder a 'waste spark.' I can post a diagram of the spark plug wiring scheme in the files section if there is any interest.

While we got rid of her car when the rear end rusted out, I found I didn't quite delete all the service data like I thought I had.?

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I always wondered, still do, what effect the "wrong" polarity voltage on the spark plug tip will have.

Based on what I learned in a physics class a lifetime ago, you'd want the center terminal of the spark plug to be positive because it will arc at a lower voltage.

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But the ZX2 had plenty of horsepower, really way overpowered for the weight of the car, it'd get away from you if you had a heavy foot on take off. The cops in central Kentucky love to write tickets for "improper acceleration" and if you lay rubber or scratch off around a cop, you will get a ticket. I always had to be extra careful to use a light foot when driving her car. And driving that car in snow was an interesting experience.?

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So the ignition system in the ZX2 had more then enough voltage/current to fire even a negative center terminal spark plug.

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I saw the spark plugs in another ZX2 that hadn't been changed since the car was new, the ground electrode had been eaten away by 280K miles of "sparks" and the car still ran fine. The center terminal was also eaten back but not near as bad. Frankly if you'd shown me the spark plugs I'd have bet the engine they came from would not run. The ceramic cone looked nice and grey like they are supposed to. Replacing the plugs didn't make any noticeable difference in performance horsepower wise, but did improve gas mileage about 10%.

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So I guess the ZX2 had plenty of high voltage.

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Back in the late 1960s Champaign offered an odd plug with no bent over ground element. The spark traveled directly across a ceramic plate. My father tried them in his Chrysler Valiant. They seemed to work OK.?

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To check polarity of voltage feeding center element of spark plug

"use a soft graphite pencil (the softer the better) sharpened to a fine point. Disconnect a spark plug lead and remove the resistor plug cap so you have the bare HT lead conductor exposed. Hold the end of the lead close enough to the cylinder head for a spark to jump. Put the tip of your graphite pencil in between the lead and the head. Now with your 3rd and 4th hands (you will need helper) switch off the workshop lights and crank the engine. A spark will jump from the lead to the engine via the graphite pencil. In the darkened room, you should be able to see a flare (of ionised graphite) from the pencil tip. If the flare is toward the cylinder it indicates correct polarity. A flare toward the HT lead indicates reverse polarity."

From

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He notes that with the one ignition pack for two spark plugs one will be fed reverse polarity.

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Enter "what effect does pressure have on an electrical arc?" into Google and the AI bot has a nicely detailed explanation of the relationship between air pressure and the voltage to strike an arc.

I'd enter the direct link but is way too long and unwieldy and you can'c chop any of it out.

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Most gasoline engines have at least a 6:1 compression, most car engines probably have 8:1 and motorcycles have up to 12:1 for high performance "donor cycles." The Honda CB/C: 250/350 had a compression ratio of 9.3:1, the higher the compression ratio the higher the octane required. That's why racing motorcycles often use aviation gas...it has a much higher octane.

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Thus ends today's lesson on ignition systems, which I'm sure bored everyone to tears.

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Too bad this isn't 1960 and the Ed Sullivan show was on Sunday nights because the presenter is a much better comedian then engineer. His escapades would go over quite well with dancing bears and guys who spin plates on poles.

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However, he'd be tossed out of, and banned, from any university electronics laboratory for his foolishness.

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Suggesting using a resistor divider to reduce direct AC mains to feed a microcontroller is downright stupid verging on criminal.

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The way you measure ultra high DC voltages is with a electrostatic voltmeter.

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I bought a ESH model 600085 at a surplus sale for $5 that will measure 5, 10, 15, or, 20kV. Other then testing it in a real electronics lab, I've only used it once to measure the high voltage for an night scope (starlight scope) tube power supply.

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However for measuring the ignition voltage for anything other then a model airplane engine, which uses a 1.5V glow plug, one needs something like either the?gizmo made from plastic with a screw adjustable gap to measure the ignition/spark voltage, however this measures the voltage under "no load."

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Or the Leslie unit I mentioned earlier which actually shows you how strong the arc is with the engine running.

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When the cylinder rises during the power stroke, the 'air' pressure increases, if you have an engine with 10:1 compression, then your pressure when the spark occurs would be 10 X 14PSI, 140PSI.

140PSI = ~ 7240 torr.

see the graphic at

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The chart maxes out at 720 torr and shows the voltage required to be in excess of 70,000V. [I have doubts about this graph but don't feel like working out the math this morning.]

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Wiki goes into Paschen's law in quite a bit of detail.

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All of this is interesting from a theoretical angle, but the practical aspect is simple:

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"Just because your ignition system produces a spark/arc in a spark plug outside the engine does not mean it will produce a spark in the engine under compression."

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I've seen this in Honda CB350s, 1968 and 1969 stock VW bugs. The ignition coils in the Honda twins was located where they were heated by the engine, I never had a hard failure but I could tell when it was time to replace one when the power dropped way off and the engine sounded funny, both ignition coils never failed at the same time so the power was really imbalanced between the cylinders. I limped home from Mason Ohio [near Kings Island] to Lexington one night sweating "Will I make it home...."

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VW coils just failed for the fun of it. West German manufacture wasn't quite as perfect as they'd have liked for you to believe.

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I learned more about ignition systems then I wanted with those vehicles. I've never had an ignition coil/system fail in

Chevy Nova [My wife's when she was my girl friend. Car died when someone t-boned it in a parking lot. Bummer]

Subaru [two cars, 1980 station wagon, 1984 compact]

VW transporter van with Porsche engine

[I have no idea why a very similar engine never experienced a fuel pump failure or ignition coil failure.]

Toyota [3 cars]

Ford [1 car we put over 200K miles on.]

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I did have the ignition coil fail on a Plymath Omni, about a month before the engine decided to emulate a grenade. [Way too exciting at 80MPH on the Mountain Parkway at 3:00AM. You've never lived until you have parts of the engine come through the hood right before your windshield is covered in oil and coolant. Loads of unfun.]

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[I always carried a spare fuel filter, fuel pump and ignition coil. <and fan/generator/ belt> for the VWs. I've changed the fuel pump and ignition coil by feel in pitch black dark out in the boonies more times than I care to remember. Yea for a person who collects flashlights it was silly not to have one, but the batteries were always dead when I needed the light. I finally got smart and made a 12V trouble light with 20 feet of wire, a fuse and power port to the battery and never had a fuel pump or ignition coil go bad again.]

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A dying ignition coil can produce what appears to be a very strong arc in a spark plug outside the engine and not work or sort of work in the engine.

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Sort of work to mean, rough idle, stalls when you give it gas.

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Modern automotive/light truck ignition coils are extremely reliable.

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Briggs and Stratton lawn mower ignition coils not so much.....

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Real mechanics with real garages with real test equipment designed for cars often have specialized oscilloscopes that will display the ignition voltage. It's worthwhile getting a glimpse if you have a friendly mechanic.

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I would never consider trying to build an adaptor to allow me to see the ignition voltage waveform on my DSO. One could probably salvage the ultra high resistance resistors from cold war civil defense ion chambers and add plenty of diode protection clamps but I'll let others fry their DSO when the Leslie ~$20 device works very well.

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BTW I have no business relationship with Leslie, I'm just a very satisfied customer.

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For the really brave...[or foolish]

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I have no idea if the device will work or fry your scope.

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Ya pays your money and takes your chances. [not me.]

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Oh changing the two drive belts was amazingly easy, took two hours, mainly because I removed the pulies and lubricated them [one bearing will need replacing sooner then later, it's on order] and cleaned the space under the deck, used a wire brush to remove some rust and painted it with a high quality epoxy paint, and I blew dead grass bits out of 'everywhere.' I also added a kill switch to the seat, get out and the engine shuts down. The twins are a bit careless. I do not want anyone to lose a foot. [Yea I'm paranoid.]

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