THANKS For posting that link, leaned a little from it. If you got the gap set right on the plug and you have good spark with a tester you don't need to measure it. Like in the article it takes twice the voltage to jump two gaps ( tester and the plug ) as it does the spark plug............................Curt
I used to check for spark when an engine would not spark using my clamp on inductive timing light to see if it would flash the light. For a magneto engine I think I had to attach a 12 volt battery to the light as with an automobile, but the battery had nothing to do with the spark from the magneto it was just for the light.
I never really checked if it could flash the light some how, but not actually spark the plug. Inside the cylinder under compression as it is different than when outside resting on the side of the plug on ground.
The point to this is I think if the plug does not spark, because the gap too big, fowled, or shorted, would the light still flash anyway. If it did that would at least tell the coil or magneto is working.
On a multi-cylinder auto engine I would put it on the distributor center wire just to check to see it flash.
I had times when it was not good enough to see the timing light check the timing. It was not really good enough for a very strange usually not too often to have happen problem.
The coil had a bad crimp connector on my automobiles 12vdc connection. What it did was allow the engine to run and sometimes run rough and stall. I had thought since I saw the timing light show the timing mark off, I adjusted the cap to set the timing.
But after a while it still ran rough and stalled. The cap was not loose and did not move. It was the resistance in the bad crimp connector changing resistance from engine vibration. So for that the 12vdc at the coil that changed the timing. I got a new crimp connector and soldered it also. No more problem.
As for the spark tester maybe I see getting one as they are inexpensive. I have used high voltage 100:1 probe on oscilloscope (use storage scope to capture pulse) and look at stuff in TV HV circuits, but don't have that equipment at home. Some scopes would need more than 100:1 so maybe 1000:1 probe if it were necessary could be used. A DVM that reads HV that high still would need a HV probes as regular leads would not be safe.
