Long arcing sparkplug

What is the problem when a plug is fouled and you hold a gap on the plug wire to plug and it starts firing? I have seen the answer in previous forums but cant find it now.
Thanks in advance
pete

Sounds like time for new plugs, had a set of champions that one did that. Put a new set of Autolites in over a year ago with no problems since. Scott

Most often oil is reaching the cylinder. As to why,hard to tell without hands on but if you are trying to run down the source of problem add some more info including;
Is all other plugs look and fire good.
What kind of engine are we talking about.
Have you run a compression test? Results?
Any smoke and where from.

JC Whitney used to sell little devices that would do that for you, force an external spark. I am sure someone else may have a better explanation but I think it works like a capacitor, the external spark gives it a boost. You need new plugs, maybe a little hotter.

has to do with the the laws of electricity, when the spark has already jumped a gap equal to or greater than the the plug gap, it will jump the gap on the plug. I have heard of people putting a soybean betwen the end of the plug and the plug wire on a plug that kept oil fouling to keep it from missing. I am sure someone can give you a better more technical answer to your question .

When a spark coil is saturated with magnetism and the points or electronic switch break (mag or distributor are the same in this regard) the magnetic field collapses. as it does so the voltage in the spark wire/s builds up until the voltage at the widest gap is sufficient to start a cascade of electrons making a plasma trail of super heated air (spark).
If the plug is fouled, the voltage can bleed off and or the spark is along the surface of the insulator to ground, and not out in the fuel/air mix.
When a person puts a wider gap in the spark path, the voltage must rise above the gap resistence and when it does it is far higher than it was just with the fouled plug. This higher voltage jumps that gap and is high enough to jump to the ground electrode and fire the cylinder.
Cleaning the plugs (heating it to near red with a propane torch on a fire brick to burn out fouling can work. New plugs are a better choice. Fixing the engine to reduce oil consumption is best. Jim

As the magnetic field collapses, a high voltage is induced in the secondary windings of the coil. The voltage build up is not instantaneous but rather takes a very short bit of time. This time delay can become a problem when the plug is fouled with conductive deposits. The voltage can bleed off preventing it from reaching a level sufficient to jump the spark plug gap.

Creating a "booster gap" allows the voltage to build up across the additional gap and prevents bleed off across the fouled gap. Once the booster gap becomes ionized, the spark jumps and "appears instantaneous" at the fouled gap. Since there is not sufficient time for the voltage bleed off to occur, the fouled gap becomes ionized and the spark jumps.

In efffect you have reduced the "rise time" of the coil.

Professor Jim has a great explanation with which I agree (as we usually do) I will just repeat the concept in my own words.

When oil or excess raw gas or carbon (fouling) accumulates on the plugs ceramic/porcelain insulation, such creates a conductive shorting path for current flow. Normally in a non fouled plug when the coil fires its HV primary voltage begins to ramp up and if theres no bleed off current path (like fouling creates) voltage will rise to maybe 8,000 volts and suddenly theres a high temperature arc of current across the plug gap which ignites the fuel air mixture.

HOWEVER if the plug is fouled when the coil voltage just begins to rise all the coils stored energy gets dissipated as current through the fouling material (I squared R) and it never rises high to a point that theres an arc of curent acxross the gap, the voltage only rises to a relatively low value insufficient to arc the gap since theres that fouling material conducting current right from the get go.

HOWEVER if that shorting path is more out of the circuit due to the added wider air gap (like if plug wire is held away from plug) voltage is allowed to rise to that higher value sufficient to arc current across the gap and she goes ahead and fires.

To reduce fouling one might run a leaner fuel/air mixture (reduces black sooty carbon deposits on plugs),,,,Run the engine at a hotter temperature,,,,,,,,Use a higher heat range plug,,,,,,,,,,,Fix the oil burning problem (rings, valve guides etc)

Hope this helps

John T

sooo, my question is will running a wider plug gap have any benefit to fire fouled plugs???

Lots of answers here regarding a fouled plug bleeding off coil voltage, but thats not true there is a small air gap between the rotor and the distributer cap post.

A wider plug gap would help extend the time until the plug fouls again, but hotter plugs would be better .

The fouling is what leaks off the voltage before it can build enough to jump to the ground electrode. The electrons see a different path and go there as though it was easier than jumping the real gap. The path is almost always (today) down the side of the porcelain insulator. Putting a wide gap to the ground electrode actually makes it easier to foul.
A good running engine with a good air cleaner. will not deposit material on the insulator. If it tends to foul plugs, it probably needs service. Higher heat range plugs can mask the issue for a while, but not if worked hard, as the plug will probably cause preignition. Jim

That fouled plug is totally isolated from the coil until the moment the rotor lines up with the post on the cap and the points open. The spark then has to jump the air gap in the distributer to get to the plug wire. The rotor is in between posts on the cap when its building voltage. There is no way a fouled plug can bleed off voltage from the coil. After the points open and the field collapses the fouled plug then effects the system.

In the 50s, Dad cut pieces of plastic folding ruler to make 'extensions" on the spark plugs of a fouling engine. Used tops of old plugs, drilled two holes about 3/8 inch apart in the plastic, and mounted them on the plugs. Plug wire went to first terminal (the old top), then jumped to the real plug terminal. Said it made for a hotter spark.

Run the coolant at 195F, lean the idle and run mixture. Use SF-2 low ash oil that is usually used in two stroke Detroits.

With respect, the rotor gap is usually in the .005 to .008" range. That gap will let the spark jump to the wire then to the plug. if the voltage is lets say 1500 volts, it will bleed through the insulator fouling to spark plug body easily. I have watched it many times on a Champion spark plug tester. As the voltage is increased by making an auxiliary gap from the ignitor coil in the tester, the weak failed spark along the insulator will leap out and jump the gap of the plug where it belongs. Jim

I think I understand your question. When I posted how a friend started a Cont flathead by slowly bringing a wire closer and closer to the plug while the starter was cranking, you wouldn't believe the ridiculing I got by all the know it alls on here.
The guy that did this with me explained it as best he could, cause he isn't an electrical neurosurgeon, just a fork lift mechanic. He said a long arc still feels the juice trying to find the highest point- kinda like lightening, when it hits, it is a higher voltage than if it was in contact, so fouling or not, the gap will make a spark and fire if there is gas.
We were dealing with a brand new seemingly dead coil, and that has been fine since that moment too, again, a jolt of higher voltage kicked it into life same time the fouled plug started firing.
Or, there was a guardian angle of Cockshutts behind us, or an invisible space alien feeling sorry for dumb humans, or I just make this $%^t up off the top of my head....
BTW, just hold the wire with insulated pliers, not bare fingers like my friend did.

You guys are woderfull,thank you for the info.I learned so much from these answers,Yes the final repair will be a ring job.Just trying to get by till spring.
Thanks again,
Pete

With all due respect and NOT to fight or argue (Im here to HELP NOT FIGHT LOL) and everyone is entitled to their opinion (lay or professional) BUT NOT THEIR OWN FACTS, as an Electrical Enginner over 40 years BSEE Purdue University (69/70) I have to disagree with your statement

"There is no way a fouled plug can bleed off voltage from the coil. After the points open and the field collapses the fouled plug then effects the system."

Sorry but I respectfully MUST stick to my and Professor Jims explanation, Champion spark plugs (they may know something about fouling spark plugs dont ya think) also agrees as they made plugs "U" or "UD" that had that extra wide built in air gap for the very purpose Jim and I discussed

God Bless you and hope you have a Happy New Year

Thanks for your fun discussion

John T

NOPE NOT much if any at all help although it probably wont hurt things. The coil will run a bit less efficient (more heat losses less efficient energy transfer) as the voltage has to rise to a higher level to jump the gap.

Fun chat, GREAT QUESTION hope this helps, see professor Jims comment with which I heartily agree......

God Bless yall

John T BSEE, JD

ok thanks john. also found out yrs. ago to pull the coil wire out some on a flooded engine while cranking to get it started.

I have a couple of questions.
Exactly which post(s) nailed it down to worn rings rather than bad valve guide seal,broke ring,a newly overhauled engine where all ring gaps were incorrectly alighned or some other problem?

I didn't and still don't know if the ignition is electronic,point or magnetio. Please explain how those in depth responses were possiable or more precisly,how in the world did you guys know what kind of ignition you were dealing with.

Most confusing to me was that I thought the root question was why the plug wouldn't fire without creating a long gap.

Please stick to layman terms,I'm no engineer and don't want to become more intangled in my Long Johns than already.

(quoted from post at 07:28:49 12/29/12) With all due respect and NOT to fight or argue (Im here to HELP NOT FIGHT LOL) and everyone is entitled to their opinion (lay or professional) BUT NOT THEIR OWN FACTS, as an Electrical Enginner over 40 years BSEE Purdue University (69/70) I have to disagree with your statement

"There is no way a fouled plug can bleed off voltage from the coil. After the points open and the field collapses the fouled plug then effects the system."

Sorry but I respectfully MUST stick to my and Professor Jims explanation, Champion spark plugs (they may know something about fouling spark plugs dont ya think) also agrees as they made plugs "U" or "UD" that had that extra wide built in air gap for the very purpose Jim and I discussed

God Bless you and hope you have a Happy New Year

Thanks for your fun discussion

John T

Click to expand...

I already have a 4 year college degree and earn a living fixing things others can't...so please Explain to me why then a fouled plug that is bleeding off voltage from the coil according to the "professionals" does not effect the other cylinders when its their turn ie. rotor in alignment, to fire. If it as you say and the fouled plug has some magical connection to the coil then 1 fouled plug should prevent all the cylinders from firing. The coil has the exact same potential for all the cylinders and each cylinder has its own circuit (wire and plug), the fouled plug is a SHORT in that cylinders circuit, After firing the shorted cylinders circuit, the points then close recharging the field and the rotor moves into the next cylinders circuit. The dead (shorted) cylinder behind the rotor is now isolated from the coil.

The path doesn't include but one plug at a time (on this engine, (several engines have a wasted spark on one plug of 2 firing at once from the same coil)
Thus there is only one path for volts. All open gaps in the spark path are additive. Thus if the rotor gap (as below) was 1500 volt and the spark plug in fuel and air was 6000 volts, it would take 7500 volts to jump. If the leaking plug's fouling was draining at 1000 volts and going to ground, the total would be 2500 volts and never jump the real plug gap. If an added gap forces 7000 volts to jump it, the plug will now get a zap of that voltage plus the rotor gap 1500. 8500 volts now does jump the intended gap and the plug fires. Jim