Spark Issue, What's causing this ?

sunsettractor · Jun 14, 2015

I was out playing on my 300 Utility, overall a pretty solid and very handy tractor. Engine just started missing, first thing I did was pulled spark plug wires one at a time and found the guilty cylinder. What is strange is that when the plug wire is firmly pushed all the way into distributor cap, engine misses, if I pull the wire completely out of the cap, the miss is the same. Then... If I hold the wire close to the distributor cap so that the spark jumps across, the engine smooths out and miss goes away. Push the wire firmly into cap and misfire comes back. I installed a new set of plugs and wires, gapped points, the cap looks new inside, no obvious wear or cracking, no difference at all. Is there any one thing that would cause this? Any help here much Appreciated...Tom

MARK ROBKE · Jun 14, 2015

Replace that plug-it is fouled.When you hold the wire so the spark has to jump,the coil is producing extra voltage and fires the fouled plug.Mark

Dick2 · Jun 14, 2015

Could be the cap is faulty. Had that happen a couple different times; after everything else didn't correct the situation, a new cap made it run right.

KenL · Jun 14, 2015

Had that happen on an international once right after a tune up. Ended up taking the brand new champion plugs out and put the old AC plugs back in. Ran like a charm.

John T · Jun 14, 2015

Not being there I will have to give my "best guess"

That is, the spark plug in that cylinder is fouling out and I would replace the plug (maybe all while you're at it)

Here's what happens, when the coil fires the voltage is NOT instantaneously at say 10,000 volts it ramps up until its high enough so the spark arc jumps the plug gap. HOWEVER if there's a shorting fouling path in the plug (oil or carbon etc), as coil voltage raises current starts flowing through the shorting/fouling path WITHOUT ANY SPARKING and the coils HV secondary energy all gets depleted as I Squared R current through the fouling/shorting path with NO SPARKING. If you introduce the extra added gap, current doesn't get the chance to flow/discharge via the shorting fouling path. NOTE Its NOT that theres necessarily more energy, the coils HV secondary has a fixed finite amount when the points open, its that the shorting path isn't there to bleed off the energy prior to a normal spark condition.

There could be a shorting fouling path at a place OTHER THEN the plug, but Id try a new plug first. If its not that Id be looking at cap and rotor and plug wires.

MY GUESS IS THE PLUG IS FOULING OUT maybe others have other opinions????

John T

old · Jun 14, 2015

Bet if you move the spark plug form that cylinder to another cylinder the moss will follow the plug. That plug is foul or just plain and simple bad.

bill mart · Jun 14, 2015

what John T said !

JOCCO · Jun 14, 2015

Agree with John T and others BUT a weak valve spring will cause these issues.

RWT · Jun 14, 2015

I'm with Dick2. It's the cap and I've seen it twice now.

RT

Deere Mark · Jun 14, 2015

Totally agree with John T. And the best explanation of this problem ever.
At a tractor show a long time ago, someone was selling a gadget that was supposed to "fix" this problem. It connected between the coil and distributor. Inside was a spark gap. $20 and I think I still have it somewhere. I bet you cold still buy one, but replacing the plugs is the best solution.

James Babcock · Jun 14, 2015

I like John T's explanation. I have used this effect as a diagnostic tool for decades. If the missing cylinder starts firing correctly when the spark jumps to the plug, then the plug is fouled. If no improvement, you have more serious problems like a stuck valve or worse.

JD Seller · Jun 14, 2015

JohnT has the reason it is firing when held away. They used to make spark plug boots that held the wire away from the spark plug to do the same thing. I also have seen guys take a piece of corn cob and hollow it out an put it on the spark plug the stick the wire into the other end to make a gap to do this. This would make an engine that was burning oil take longer to completely foul out the spark plug.

Steve@Advance · Jun 14, 2015

Tom said: "I installed a new set of plugs and wires"

That would have been my first plan too. But it didn't fix it...

Several good theories here, John T is correct with the voltage increase being able to fire a simi-fouled plug, but the plugs are new...

Take a look at the plug that is misfiring:

Possibly the new plug is defective, switch it with another plug, see if the problem follows.

Take a look at the plug, is it gaped properly?

Is it already fouled? Bridged with a piece of carbon? Oil splashed? Coolant? Possibly there is a problem with the cylinder (hopefully not, this is just an idea). If that cylinder was run dead for a while, it may have carboned and oiled up the combustion chamber. Once it started hitting again the carbon and oil were splashed on to the new plug. If the head gasket is leaking the plug may be getting water on it, grounding it out. If it is already oil fouled, try installing a non-fouler on that cylinder temporarly to get it cleaned up. Then maybe it will be ok once it's dried up, but there is a posibility there is a compression/oil control problem.

Still a possibility the cap is cracked, leaking voltage to ground or between terminals.

Let us know what you find!

sunsettractor · Jun 14, 2015

Good News!
I had a new d21 champion, put it in and it runs fine. Most of the suppliers I use offer a 386 autolite for this application, the plugs I installed were from an ebay supplier and are 3116 autolites, I'll probably put a fresh set of 386 autolites in it and see what happens. The D21 champions I have are used in my IH "H" which is somewhat of an oil burner, but they work well and have lasted a long time. This is a new for me people, I have had a lot of fouled plugs over the years, mostly oil fouled, I've never ran across this issue before. I did not know 'til today that a fouled plug could fire jumping across a gap like I just saw happen. You can see the plug is pretty carbon'd out, the tractor was real rich for a while, I had made a couple adjustments on the carb,seems to be better now, these plugs only have about 4 hours on them. Thanks so much everyone for the timely replies, this website has always been so helpful, your time and effort are always Greatly Appreciated, Thanks Again...Tom

JMS/.MN · Jun 14, 2015

In the 50s Dad made some of those gappers from plastic rulers or masonite...drilled two holes about a quarter inch apart, used a stud from an old plug in one of the holes, and slipped it over the engine plug. Spark would jump across. Next winter, overhauled the engine.

Bob Bancroft · Jun 14, 2015

Same here. I remember seeing those things on the WD and asking about them. Then it got overhauled at the local dealer. I believe they put an "M&W" kit in it. My cousin still has the tractor. The engine hasn't been apart since my youth.

sunsettractor · Jun 14, 2015

Thanks for the reply Steve.
This tractor is fairly new to me, purchased it off craigslist for $1500.00 and really havent done anything to it yet. Other than the plugs, possibly the wrong ones and the spark plug wires. Its a strong runner, was full of clean coolant when I got her home, hasnt used any water or oil in the short time i have used it. Had a few issues with hydraulic leaks, got the wiring hooked up correctly, 6 volt genny charges now. Havent really had any extra money to spend on her, but down the road I plan on a carb rebuild, full tune up, and taking the hydraulic reservior apart to service the pickup screen, replacement of all the hyd valve orings, ect, ect. You know how it is when you buy someone else's old project, anything is possible. But I am happy now, I get to go back to playing in the dirt on all four cylinders again, Thanks Again...Tom

GordoSD · Jun 14, 2015

Mixture too rich, and plug too cold.

John T · Jun 15, 2015

You're welcome, thanks for the feedback. INDEED a fouling plug appears to be the problem and by adding the extra spark gap you described, the plug can still fire for reasons I explained below.

NOTE ABOUT PLUGS (AS I recall but I didn't look it up again and I tend to forget grrrrrrrr so no warranty)

Autolite 386 is a RESISTOR PLUG
Autolite 388 is a slightly "Hotter" RESISTOR PLUG
Autolite 3116 is a NON RESISTOR PLUG in the approximate heat range of those above and is what Id definitely recommend when a
Magneto Ignition is used.

UNLESS I remembered all that bass ackwards (resistor versus non resistor plugs)

A carbon fouled plug (gray sooty) can be the result of an over rich fuel mixture or running at low engine temperatures, while if the coating is more of a black shiny coating that may be caused by an oil burner.

To help reduce plug fouling it helps to NOT run over rich,,,,,,,,,,operate at engine temps like 170 minimum up to 200 degrees, avoid long idle periods.

If I were using a Resistor Plug, Id opt for the hotter Autolite 388 but if a NON Resistor Plug Id use 3116

Hope this helps

John T

dr sportster · Jun 15, 2015

That is an old two-stroke trick to unfowl a plug [for dirt bikes].

Texasmark1 · Jun 16, 2015

Well done John.

Spark Issue, What's causing this ?

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