Back in the olden days, spark plugs were indeed just two wires arranged in
close proximity so that a spark of a few thousand volts would jump between
them. The wires were supported in ceramic insulation so that the
conductance of the supporting material would not drain off the electrons
(voltage) faster than the ignition system could supply them, thus allowing
the voltage to build up to the flash over level quickly.
Now, in the modern era, spark plugs are still just about that, but many have
resistance built into the "wire" so that your neighbor won't complain that
his "Must See TV" program is full of snow when you run your mower. This
resistance can fail, and make a sort of series gap inside the plug that can
require very high voltage to make it fire. That would be one kind of
internal failure.
Secondly, the ceramic insulation can become impregnated with carbon
particles that make it more conductive, thus draining the precious little
critters (electrons) off to ground before enough can be herded toward the
tip.
Third, the shape of the electrodes can become rounded off. [It's beside the
point, but this process of spark machining away metal is called "eloxing" as
I recall, (I'm not sure of the spelling), but we used it for precisely
machining parts for some of the early space hardware.]
Round electrodes make a larger gathering place for the little critters, and
they will sit there if they have room, rather than taking the fatal leap to
the other electrode. The result is a higher firing voltage, putting the
strain on all components of the ignition system. The most notable thing to
suffer here is the arm and shoulder of the poor boob who is trying to pull
the rope fast enough to get a good enough spike of voltage out of the
magneto.
Forth, the gap gets wider by the same process as above, and the gap is
critical for starting if the voltage is marginal, as it will be on any rope
pull started device.
The bottom line is: if you could perfectly reshape both electrodes in a
spark plug, set the gap back to specification, and clean all the carbon out
of the insulator, and if there is no failure of the resistor inside the
plug, there would be no reason to replace the plugs. I have a "Vixen" sand
blaster for cleaning plugs, and it seems to rejuvenate the insulator pretty
well, and so for some of my old cars for which plugs are very hard to find
(the old 10 MM plugs for instance), I do exactly the above, and have gotten
away with it for years. But this is a lot of work, and if you can buy new
plugs for a few dollars, I think that makes more sense.
Dick Benjamin (who moved to the desert in 1979 so that I would never have to
start a lawn mower again!)
----- Original Message -----
From: Crownking <>
To: <mailing-list@xxxxxxxxxxxxxxxx>
Sent: Tuesday, May 21, 2002 9:25 PM
Subject: IML: Spark Plug Experience--Dick Benjamin?
> Hello All:
>
> The incident herein intrigued me and has Imperial implications but
> the experience was not wholly Imperial. The long and short of
> it--what about a spark plug makes it a wearing part?
>
> My one year old lawnmower cut out while mowing the grass the tenth
> time this season. It would start once every hundred or so pulls but
> run for only a minute or so and badly at that. I cleaned the air
> filter to pristine. I tore the carb apart and cleaned it up although
> I was doubting old gas as a contributor since we have run the thing
> so much this year. The thing would not even start reliably on
> starting fluid. I tried it without the air cleaner to no avail. I
> cleaned the spark plug with carb cleaner followed by brake cleaner
> after filing the electrode with a teeny file and cleaning up the
> other side with fine sandpaper. I did not check the gap since I did
> not know what was correct and believed this could not be gap related
> since it ran some of the time. Nothing worked. I deduced that it
> had to be either a loose ground owing to the silly safety cutout or a
> bad magneto which is highly unlikely given its age. With Moron's
> assistance a test for spark was devised in which we would ground the
> plug and try to start it in a dark garage--if you want to amuse
> yourself just try holding a mower still enough to check for spark
> while pull starting it. Prior to making that drastic and potentially
> dangerous test, I decided to replace the plug and learned the correct
> gap in the process. The mower starts first time every time.
>
> Here is the question: Isn't a plug nothing more than a wire with a
> hole in it through which electrons jump thereby causing the necessary
> spark? What can possibly wear out on one? I dutifully replace my
> plugs in my cars as recommended but always feel like a sucker doing
> it and have never experienced any change in performance after having
> done so. This mower experience has opened my eyes though-- I wonder
> if there are a few Imperials out there pronounced DOA that are
> suffering from nothing more than a sick spark plug or two.
> Dick--what the heck should I know here? I hate it when the solution
> to a problem is not chemical in nature--most solutions are!
>
> Let the snickering begin....
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