Hi.
Diagnostics checking if a spark plug gets its high voltage on a mower is very difficult under sunlight as a typical neon monitor lamp is not bright enough.

I would like to explore how to make it audible; a piezoelectric element 'beep' at every time it is supposed to fire. Half a second beep is plenty enough to tell the tale.
It would be nice in parallel to the spark plug, or in series to it.
The thousands of volts would have to be limited/divided to preserve the piezo element health, but that won't beeeeeep...

Alternative, a loud 'clack' at every time spark occurs. Loud or distinctive because there is noises from cranking by pulling the cord.
That, I think I can do it.

Well, shoot what methods would you try...

 

What about putting a slip of paper in the electrode gap and noting if it gets burnt by the spark?

 

How about gluing a photo sensor (I.e., photodiode) to the neon bulb and encasing it in black paint or silicone. Then, use the photo sensor to trigger a piezo buzzer.

 

Use an umbrella to shield the neon from the sun.

 

My SUN timing light uses a clamp on magnetic pickup the is triggered by the spark plug current. A simple clamp on magnetic pickup could work. The pickup could drive the base of a transistor which could drive a piezo sounder, perhaps a Sonalert, or a similar device. And it would not require any tampering with thr plug wire. The big advantage is not needing to deal with the high voltage.
The disadvantage is that a fouled plug will also show a current pulse, thus it is also needed to do a resistance check on the spark plug. A good plug will register open or at least a megohm, while a fouled plug will show a much lower resistance. That was my simple and easy way of checking plugs on a car with difficult to access plugs. It works very well.