The Stop-Gap Cure For Whiskered Plugs
The Stop-Gap Cure for Whiskered Plugs
Doug Richmond
Sooner or later it happens to just about all of us. usually when we're 'way the hellan' gone out in the back of nowhere.
The bike has been running along beautifully, God's in his heaven, all’s right with the world and then all of a sudden the engine begins cutting out like mad, especially under WFO conditions.
The obvious drill, of course, because it’s easy to do, is to stop and take a quick look at the sparking plug. Nothing there; the plug is the correct rich brown, but inasmuch as it’s out and on the ground the logical thing to do is to screw the spare into the empty hole.
Whereupon the bike starts at the first prod and then proceeds to pull like a Caterpillar tractor, run like a watch and generally live up to the promises of the advertisement that touted us onto the machine in the first place.
But a short time later it repeats the performance, and as a fresh spark plug cured the problem the first time we figure it just has to be ignition troubles again. As we’ve used up the only fresh plug, out comes the trusty Swiss Army Knife and we poke and probe down among the electrodes in an aimless manner. Half-heartedly the ‘cleaned’ plug is replaced, and the cycle takes off like a Concorde leaving Heathrow. And the whole performance takes place again. And again. And again.
Which leaves most riders scratching their heads.
It’s happened to me many a time. If you stick with motorcycles long enough, especially two-strokes, it will happen to you too.
The mysterious and annoying miss is caused by the infamous whisker, an almost microscopic growth that bridges the gap between the center and side electrodes of the spark plug. Occasionally it is large enough to stand out like a pregnant prom queen, but lots of times it’s so small it almost takes strong light and a 5X magnifying glass to make it out.
Electrically, the whisker forms a highresistance connection across the secondary of the ignition coil. That bleeds off just enough of the high voltage so that not enough remains to create the spark necessary for ignition, exactly the same as a blackened, oil-fouled plug. Missing often occurs only under open-throttle conditions because the higher the pressure in the combustion chamber, the more voltage it takes to make the spark jump the gap between the spark plug electrodes.
Whiskering can strike one out of two identical models of motorcycles and not the other, even though they are using the same gas, oil, and spark plug. There are many theories about the cause. I've heard
it blamed on improper oil, bum gasoline, wrong temperature range plug, worn rings, engine run too hot and engine operated too cold . . . you name it.
But after an exhaustive amount of experience with whiskered plugs—some which was exhausting—I’ve come to the conclusion that although any and all of the above may be the proximate cause of whiskering, the underlying problem is essentially a weak spark. Not the kind that won’t ignite the mixture, but the insidious sort that causes hard starting now and then, and maybe a bump start occasionally when all else fails. It also causes frequent plug changing, as the owner learns to cure the problem the parts-counter way.
My theory goes like this: A cycle with really hot spark can fire the mixture even a bit of whiskering is present. The combination of ignited fuel plus fat, hot spark will burn off a thin whisker before it has chance to grow to power-sapping maturity. But a weak spark fritters itself away leaking through the whisker and in the end is unable to create the self-cleaning situation described above.
Under some circumstances, the cure obviously to track down the reason for weak spark and fix it. That’s fine when you're at home on Saturday with the w hole day free and the neighborhood shop and its fancy diagnostic equipment just down the street. Or when you've managed straggle home in 10-mile hops and you’re ready not to have the problem again. When life arranges itself this neatly, we’ll be able to fix bikes that way. Meanwhile. we need a way to make this weak spark and semi-fouled or bridged plug work at least well enough to sputter back to civilization.
My cure began when I remembered that the old-time mechanics, back in the days of gummy oil and before spark plug manufacturing had become a science, used to cut the high-tension lead in two and tape it back together with a slight gap. say 1/16 in. or so. This way the spark would have a chance to build up to the point where it would suddenly leap the extra gap. and because of the instantaneous high voltage, also lire across the spark plug points.
The first time 1 put the trick to the test was on a Matador between Villa (now Ciudad) Constutión and La Paz. where even the jack rabbits have to carry rations. I'd been fouling plugs about every 8-10 miles, and it soon became obvious that at my current rate I wasn't going to make town for about 10 hours. And it was getting along toward dusk already.
So as a last resort I cut a Bic pen in half, and the spark plug lead ditto, and used the pieces of pen as a sort of splint to hold the gap spacing. After tightly wrapping the whole schmeer with black electrical tape I put the lead back on the plug. Because this Matador was extremely weak in the ignition department under ideal circumstances 1 didn’t even try to kickstart. Instead I turned it around and ran it as far as I could up a little hill, then turned it around and bumped it hard as I could. The bike fired immediately and didn't make another bobble until we pulled up in front of the Hotel Praco. In fact, the gapped and taped spark plug lead was working so well I ran it another 11 miles without a plug change, with every start a bump start, of course.
Since that day in Baja many years ago I’ve had occasion to use the gapped-lead trick several times, always as a last-ditch remedy, and it has yet to let me dow n.
There's also a pre-trip solution; if you think you might get whiskered some time but don’t yet have a weak spark to fix. there’s a gadget you can make and stowaway for the future.
This trick began as something else. When I wanted a handy means of limiting voltage on snap-action magnetos, I soldered a battery clip onto one of the flats of an old plug lying around the place. (We all surely have at least one of those.) Later it struck me that this modified plug could become a built-in gap in the high-tension line, just like the gap you get when you cut the line and splice the two ends slightly apart.
Simple. Pop the spark plug lead onto the auxiliary plug, then snap the soldered clip onto the plug in the engine. You have in effect two plugs wired in series, and you've got the extra gap needed to build the voltage to work despite the w hisker. Looks funny but it’s a helluva lot more convenient than the tape-and-splice technique.
