Spark plug wire issue

This one has me a bit baffled. I had one of my cylinders not firing from time to time for reasons I could only figure had to do with the fact that the wire going to it was cracked and in need of replacement. So I bought a new set of NGK wires and installed them last night figuring that all was well, its not.
The original wires were the the big black type with the metal collar that captures the base of the spark plug. I noticed the original problem would subside if I pulled the plug wire plug off and did not let the metal collar contact the plug base. I could hear the spark jumping from the plug wire plug, then engine would run just fine (I had to ride home this way), but as soon as the metal collar touched the plug it would cut out. Why? I figuered it was due to some internal grounding issue due to the cracked wire.
So the NGK do not have any metal in them other than the element conducting to the plug at the top. This would seem to hae solved the problem, but it didnt. Now, the other side is doing this wierd behavior. The other cylinder was working initially this morening when I started my bike. All sounded good and then it started running on one cylinder. AHHH! Nice day, and now I am late for class. I removed the right wire (original one withour issue) while it was running and there was no change, remove the left and it died. Ok, I plug it all back in, start it up and the problem persists. I pul the right plug again and notice the same behavior I used to get with the old plugs; if I pult the plug wire plug away from the spark plug to where I can hear the spark arching to the plug teh engine jumps to life and runs like it should. WTF? WHY? This makes no sense to me.
How can inducing a bigger arch in the plug cause it to actually fire? The only thing I can think of is that I have the wrong resistance plug wire set, but I bought them from Bob's and they are 5K ohm (which I though was the higher resistance any way).
Help!

DrEvil said:

How can inducing a bigger arch in the plug cause it to actually fire?

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Because it increases the voltage.

Ok, that does make sense. Increase resistance = increased voltage (E=IR)

So, what ailment would cause these symptoms? Is my coild dying?

Possibly a coil problem, possibly a plug problem, possibly a mixture problem. It takes more volts to light off a bad mixture. Coil seems least likely, as it does create the required voltage when forced to.

I am leaning towad plug, but I pulled and checked the plugs and cleaned them. they looked fine. The mixture would not be my first guess as the problem modulates with the plug wire and has since shifted sides. The right coil was fine until I switched to the new wires. Plugs are an easy switch, I guess I'll try that next, but it still doesnt make sense. The mixture would have had to have switched from good to bad and back sporadically to get this result, no?

Could it be that my gaps are wrong on the plugs? Bigger gap = higher resistance, smaller = lower?

Should be 0.8mm (.032 in) IIRC.

The switching sides thing plus the new wires points to plugs or mixture.

Um pull the plug lay on cylinder, crank with ignition on, see if plug fires. Had the same kind of thing happen 4 years ago, liked to drive me nuts. Would not fire and when I got rough with it ( tried to rev it hard it backfired ) Turned out to be a condenser. Of course that was after checking points, pulling carbs down ( thought it was a diaphram ) and finally just taking a shot in the dark.

I would also add that when testing, don't pull the plug wires off the plugs when the engine is running-- this is not good for the coils and can be fatal for some electronic ignitions

rig up a way to ground/short the plug to the head. see photo

iirc this is a 3" long, 3.5mm?? bolt with the head cut off. and the adapter that comes with new plug

screw it on to the plug and then fit the plug wire on it. you can then ground the plug to the head using a large plastic handle screwdriver.

this is usually used for tuning/balancing the carbs.


hope that helps

AND THE WINNER IS.....plugs....and mixture(?)

I got home and removed the plugs to find them to be very black. This I realized could be due to the fact that two things happend this weekend:

1. I was leading a friend of mine to and from a swap meet and AX on my bike. He was driving a 74 bus towing a 914 so slow would not begin to explain how fast we were going. I probably should have had the rpms higher to bun the gas better.

2. While at the AX the bike started acting goofy again (had not yet changed the plug wires) and I had to ride it home = soot up some more.

So, I cleaned the plugs, installed them in the plug wire plugs and tested teh spark (Ohhh, pretty). Installed them completely and they ran fine, just like before. So I rode to the parts store and got some new plugs, just because. When I removed the old plugs again I noticed that where I sanded the electrodes was brown (good), but the rest was getting black again.

Is it still probable that the mixture is too rich? How would I know? (other than the plugs)

Thanks for all the input!

lkchris said:

Because it increases the voltage.

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E=IR By increasing the R one also reduces I so voltage does not increase unless of course I could remain the same which it does not

Red100RT said:

E=IR By increasing the R one also reduces I so voltage does not increase unless of course I could remain the same which it does not

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Yabbut, the laws of physics say that the secondary voltage of a transformer relates to the primary voltage of a transformer by the ratio of the windings. Ain't nothin' in there about resistance in the circuit. An ignition coil is a transformer. Fact is the voltage CAN'T change (unless either the coil is defective or the charging system or battery voltage changes).

What actually happens when you "double spark" is that when the points open and the primary field collapses and induces the secondary voltage, you have forced the system to reach a higher voltage level before the air in TWO gaps ionizes and allows the plasma (spark) to form. What this means is that the spark RISE TIME will be shorter, spark duration will be shorter, and the spark itself will be hotter (fatter). The energy of the system is identical. It all just happens in a shorter period of time. The "fatter" spark lights a "finicky" mixture easier.

I am not even gonna dig in my brain for my electrical knowledge, it is being supressed by endocrine at the moment. BUT, the plugs were dirty and increasing resistance DID allow for it to work. Cleaing the plugs allowed for the system to work in original configuration. Nuf said for the generalities of the problem.

DrEvil said:

increasing resistance DID allow for it to work...

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Unless you realize that an air gap has INFINITE resistance (until you put a voltage across it high enough to ionize the air in the gap). I'm just wonderin' how you managed to get more than infinite resistance.

Don't believe me. Try this simple experiment. Take your handy multi meter (set to read ohms, or mega-ohms if you have such-a scale) and hold the probes so they are close but don't touch. See if the reading gets "more infinite" when you move the probes far apart.

Actually, air does not have infinite resistance. At STP it takes about 30,000V to cross a 1cm gap. Also, see lightning, Vandegraf generators or other static producing devices of high voltage and low current. Your spark plug uses air between the electrodes. A megger with enough juice would show a variance in resistance given the scenario you put forth. Also, our coils work on the order of volts large enough to cross gaps.

DrEvil said:

Actually, air does not have infinite resistance. At STP it takes about 30,000V to cross a 1cm gap. Also, see lightning, Vandegraf generators or other static producing devices of high voltage and low current. Your spark plug uses air between the electrodes. A megger with enough juice would show a variance in resistance given the scenario you put forth. Also, our coils work on the order of volts large enough to cross gaps.

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So... what is the RESISTANCE of a 1 cm (STP, non-ionized) airgap, and how do you measure it? (IONIZING it first is CHEATING!) Is the resistance of a 2 cm (STP, non-ionized) air gap twice as much?

Hey you guys. This is an American Motorcycle Forum. Speak English or go back wherever you came from

Dave H
San Antonio, TX

Bigrider said:

Hey you guys. This is an GERMAN Motorcycle Forum. Speak English or go back wherever you came from

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There, Fixed

Bob_M said:

There, Fixed

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Ok... just for grins... my ITALIAN designed and built motorcycle with the AUSTRIAN engine in it that has the GERMAN propeller-hat logos on the side and a COLORADO license plate, doesn't use AIR in the spark plug gap unless I was stupid enough to run out of gasoline. My bikes use a near-stochiometric (on the rich side) fuel-air mixture that is not at standard temperature and pressure unless I forgot to screw in the spark plug and/or have something bad-wrong like a BIG hole in the piston. Oh, and things get pretty hot there in the combustion chamber pretty soon after the motor starts running.

However none of that stuff changes the fact that air doesn't like to pass electricity without some pretty serious pre-conditioning (known as "ionization"). Regular, regulation, breathable-by-human-beings type, AIR, has infinite resistance. If not... your house would be a whole lot more dangerous place than it already is. You'd have sparks flying around between the holes in all your wall outlets and such.