mike wrote:
What can I use in place of a M-80 or S-80 neon test light to test the
ignition on an even rude 25 hp motor. What exactly are these
testers. Thanks mike.

It's a neon bulb with a resistor in series.

The neon strikes (turns on) at about 90 Volts and will glow with
very small currents. As long as there is about 60 volts at the bulb
terminals it will continue to glow, even though the current may be
only a few tens of microamps.

The current in the tester is limited by the resistor, usually about
200 k ohms in that type of unit.

The characteristics of the insulation, the resistor, the neon gas
arc and the high voltage spark make the neon spark tester unique. It
will glow when the spark is strong enough to overcome the resistance
of the insulation on the spark wire and resistor and still get up to
90 volts at the bulb, even if you hold one of the terminals instead
of touching it to the engine block.

The only truly reliable test of a spark circuit is to remove the
spark plug, connect the wire, lay the plug on the block and crank
the engine. The spark should be strong and easily visible in daylight.

Experience will teach you the relative veracity of the neon tester.
It is a quick troublshooting tool that will confirm a good strong
spark circuit, but may not tell you much about the condition of the
plug wrt wear, fouling, gap, etc. Old timers think they can intuit
compression from a neon test, but I think it's all in the scowl, and
wether or not they think they can sell you a ring job.

Terry K