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.

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

"Terry Spragg" wrote in message
ogers.com...

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

Are you familiar with the S-80 and M-80 specifically?

The reason I ask is that there is also a type of timing light that uses a
neon bulb in series with the spark wire. You pull the spark plug wire off
the plug, stick one wire of the light into the bulb and the other wire onto
the spark plug. The ignition spark then flows through the neon bulb to the
spark plug. There is no series resistor for the neon bulb.

30 years ago these type of timing lights were popular because they were so
much cheaper than the inductive pickup types. Since then, the advancements
in electronics have made the inductive pickup units so much cheaper that the
neon lights don't make a lot of sense, especially because they were so dim.

The neon lights still have one feature that would make them useful: they
don't require any power source. This is really helpful on a small engine,
such as a small outboard, that does not have an electrical system to power
the inductive pickup units.


Rod McInnis