‘72 Commando arcing coil

ILLF8ED · Jul 19, 2019

A new one for me and I’ve been around Commandos since 1973. After taking the opportunity to push the speed up on a straight freeway and successfully getting from 65mph to 90mph is a few seconds I’m declaring success on correcting the cam timing on my combat. However exiting the freeway a misfire on the left side started. Bottom line there was an arch between the positive low tension pole to the rubber cap on the high tension. I pushed the rubber cap on a little harder and rotated the coil a bit the arch is gone and runs perfectly. These are Emgo coils...not expensive. Anyone else experience this?

MexicoMike · Jul 19, 2019

There's probably a carbon track on the coil rubber cap/coil nose that is causing the arcing. Rotating the coil/pushing on the rubber cap most likely moved the carbon-tracked area far enough to where the spark couldn't jump the gap. BUT the carbon track is still there and will be the "weak point" in the secondary. Once a carbon track exists, it can't be "removed." If the track is in the secondary wire rubber cap, the wire/cap should be replaced. If the coil nose is 'tracked,' the coil should be replaced.

Craig · Jul 20, 2019

Okay .... please excuse my lack of knowledge ,what is a “carbon track” thanks

kommando · Jul 20, 2019

A track or trace of carbon, which allows electricity to follow that track instead of where you want and need it to go.

Follow the path of least resistance.

MexicoMike · Jul 20, 2019

Yes, actually quite common on older ignition secondary components - coil, distributer cap, carbon resistance wires.

Craig · Jul 20, 2019

How is it created ?

kommando · Jul 20, 2019

Spark plug carbon track. A common source for a misfire is arcing in the spark plug boot. Air can ionize and become more conductive. This can cause an arc adjacent to the spark plug insulator and eventual carbon tracking that makes the progression of the failure get worse.

Always good to look inside an old car with coils in the dark with the hood up engine running, can be quite a fireworks display.

Hence the requirement for regular changes of distributor caps and HT leads in servicing requirements.

ILLF8ED · Jul 20, 2019

Guess it’s time to replace the plug wires. The Emgo coils are fairly new. Replaced those when I replaced my Lucas RITA with a TriSpark...maybe 2000 miles ago. The plug caps are also fairly new NGK resistor type. Wires as I recall are wire rather than carbon. The arc was from the positive low tension pole to the coil rubber cap.

maylar · Jul 20, 2019

I would replace that coil. Carbon arcing is not reversible.

ILLF8ED · Jul 20, 2019

maylar said:
I would replace that coil. Carbon arcing is not reversible.

The coils are fairly new. Let me try a wire replacement first. Wondering now if an arcing coil was the reason I thought the RITA failed. Good thing I didn’t toss it.

maylar · Jul 20, 2019

The thing is that once the carbon track appears it burns into the insulation (phenolic?) and becomes a permanent path for the high voltage arc.
Avoid riding in the rain, it'll be worse when wet.

jbruney · Jul 20, 2019

The old televisions suffered from much the same in the area of the tripler near rear of picture tube. Caused quite a display when they jumped creating trash & smoke. You'll ( I agree ) most likely get another visit if you keep any of the affected items, but you may luck out on the coil...May.

MexicoMike · Jul 20, 2019

Yeah - A common quick "ignition check" we used to do was to open the hood after dark, spray a mist of water onto the engine/engine compartment and start the car. It provides an INSTANT picture of ignition system integrity. As Jbruney posted - it could sometimes be quite the light show. The goal was to totally eliminate the light show which sometimes required the replacement of all wires, distributer cap and coil(s) and it often included repositioning spark plug wires.

cliffa · Jul 20, 2019

I had it on my car ( A Vauxhall Viva HB SL90

) back in the 70's. It was tracking inside the distributor cap. The car ran terribly and fuel economy went through the floor. It was incredibly difficult to diagnose as you couldn't see it tracking. I changed the plugs, points, condenser & fuel pump and the issue remained. By this time I was convinced it was a blown head gasket or burnt valves. After weeks of frustration I was about to pull the head when my neighbor casually mentioned he'd had a similar issue and advised me to swap out the cap. I did it and WOW, really WOW what a difference, it was like I had a new car!! and the cap probably cost less that £5.

My advice - Swap out the coil, once the path is established it will find it's way back (and probably at the most inopportune moment).

Cheers,

cliffa.

cyclegeezer · Jul 20, 2019

illf8ed said:
Guess it’s time to replace the plug wires. The Emgo coils are fairly new. Replaced those when I replaced my Lucas RITA with a TriSpark...maybe 2000 miles ago. The plug caps are also fairly new NGK resistor type. Wires as I recall are wire rather than carbon. The arc was from the positive low tension pole to the coil rubber cap.

I usually go from cheapest to most expensive repair, so spark plugs, then resistor cap, then wire, then coil.

ILLF8ED · Jul 21, 2019

It turned out even cheaper. Under the rubber caps on the coils the plug wire had not been pushed in all the way and some of the metal clip was making the arc to low tension posts. Used some special pliers to get the wires all the way into the coil then use 90 degree rubber caps. Had to readjust the ignition timing as it was too advanced. Running great this afternoon.

jbruney · Jul 21, 2019

Hope it continues to do so. Mine did similar a while back and just died until I corrected the situation.

‘72 Commando arcing coil

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