Aberrent Spark form the #3

[Diastole]

I need help. I have floated this around my Model A Club and getting mixed thoughts. I wondered if the experts here had any good input.

Patient: 1930 Standard Coupe, early oval speedo. Engine overhauled in the 60's with 0.060 overbore. Top end replaced in 2024 with 5.5 head. New Autolite AT-10 plugs.

Problem: After driving car enough to warm up the engine, does not occur on a cold engine. When at idle, and near end of burning carb of all its gas, a spark jumps from the #3 bronze wire to the spark advance linkage, see photo.

Symptoms: None. Car purrs in all phases of running. Starts immediately, no backfires, no misses, runs smooth, no smoke, good power, good acceleration. GAV required to be open 3/5-3/4 of a turn until engine warms up, then can turn GAV to 1/8 from closed. Engine runs best with GUV only open 1/8 of a turn.
When parking car, plan to drain carburetor, turn off fuel, leave car at idle. About halfway thru burning procedure I hear a clicking. And the video and pic attached show spark jumping off the plug wires to the spark advance linkage. No misfire. Engine not missing, listen to engine on video here at this link.
https://drive.google.com/file/d/18w8fhaJS8mEm8G3D30IjoE2XpZdsQcF_/view?usp=drive_link

On rare occasions I have heard the clicking at a stop light, but this is only intermittent. I do not have any misfires or missing while driving or idling.

The plug is snug in its hole, no gas leak. I have not checked the plug's ceramic insulator for a crack, I have not checked the gap, I have not cleaned the plug. The compression in all four cylinders is between 89 psi and 92 psi, including #3.

Any thoughts?

Diastole in the Smokies.

 

[Flathead]

Can't you just bend the copper strap away from the linkage a little more and see if that solves the problem?

 

[JoeCB]

Well, this will be an interesting one, for sure. The electrons seek the path of least resistance, hard to believe that that large air gap has less resistance than the SP gap, even considering compression pressure. Have you tried switching plugs? different plugs?
Joe B

 

[alexiskai]

I had this happen with a Winfield head. That head moves the #3 plug closer to the advance control rod, and I started getting arcing from the plug terminal to the rod. I was able to mitigate the problem by insulating the rod, but the solution was to change to a taller/shorter (can’t remember) plug that increased the distance.

I would reduce plug gap on #3 as a diagnostic measure. Just see if it makes a difference.

 

[nkaminar]

On my car I have a split rubber tube around the advance rod. I have the split away from the brass strip lead. I also have taller plugs, Champion W16J and the rod bent away from the strap.

More: Air under pressure offers more resistance than air at atmospheric pressure. So it takes more voltage to jump the gap at the plug than it takes to jump from the lead to the advance rod. That is why it is jumping to the rod.

 

[Kurt in NJ]

what is the spark plug gap?

 

[Ruth]

Use a taller plug, use a longer strap, use modern plug wires, bend strap farther away.

I use Champion W-18 plugs with the 3¼" straps.

 

[Bob C]

The question is why is the coil putting out so much voltage.
I would look at the wire from the coil to distributor, maybe
it is damaged in the tower.

 

[Kurt in NJ]

also check resistance from top of plug to center electrode— perhaps there is a gap inside the plug too

 

[krazygimp]

I am very curious to see what you find. I inherited a 1930 Coupe a few weeks ago that has spent a lot of time sitting over the years. While it was running in the garage one night I noticed the same arcing from the number 3 plug to the advance rod just like you. As a quick fix I wrapped some electrical tape around the advance rod and that stopped the arcing but was never able to figure out what the cause was. The rod was about 1.5-2 inches from the spark plug but easily jumped that gap. I checked the gap on all my plugs and was surprised to find them all around 0.020. They are the Autolite 3076 plugs that appear to be almost brand new. The distributor had been rebuilt with the modern points and wireless bottom plate. It is running a rotor and distributor housing that appear to have brass terminals. The spark plug holes are clean and not rusted and the spark plug straps are also shiny and clean. It confuses me that the spark would chose to jump an almost 2 inch gap to the rod instead of the 0.020 inch gap at the plug.

 

[Bob Bidonde]

I had a spark arcing from the #3 spark plug to the spark advance control rod. I suspect that the cause of the arcing is the modern high voltage 12V coil in my car. To fix the problem, I added a piece of rubber hose to the control rod.

 

[Diastole]

Update:
Ignition Coil is new from Ohio supplier, not a hot coil, 4 m old
Coil wire 4 months old
6V positive ground car
Generator with third brush set to put out 7 amps
Distributor rebuilt with modern points (could not source NOS points)
No significant change on ammeter during aberrant spark
Gap was set last fall at 0.035”

Reminding everyone this does NOT happen unless mixture is leaned out and at idle.
Not while running car. And it just started 2 months ago.
The number 3 cylinder does not miss, even with aberrant spark

Plan for tear down tonight:
1) will check resistance from terminal to head and resistance in plug
2) check insulator ceramic for crack
3) will measure gap between rod and wire
4) will check SP gap
5) will check for carbon
6) plan to swap #1’s and #3’s plugs
7) try a new plug

I will take photos of everything and upload

I do not mind insulating the spark adjustment rod, but, I would prefer to find what the problem is and fix the problem not patch for a solution


Thanks for all the help
Diastole in the Smokies

 

[alexiskai]

The number 3 cylinder does not miss, even with aberrant spark

With respect, this is impossible given my understanding of how engines work. You wouldn’t get simultaneous arcs at the plug connector and at the plug. If it’s arcing at the connector then it’s not arcing in the combustion chamber. Perhaps because it’s idling the misfire isn’t apparent.

 

[katy]

New Autolite AT-10 plugs. Are they resistor plugs?

 

[ndnchf]

Get rid of those plugs and put champion 3Xs in it. Those autolites are too short causing the strap to be too near the rod. I had the exact same problem. Switched to the correct plugs - problem solved.

See my youtube video.

https://youtu.be/oAS8EAZAeGE?si=7KIGr1LoxY2safHl

 

[alexiskai]

Quick thought: the spark will be at its strongest at idle because that’s when the dwell time is the longest. A lean mixture is more likely to misfire. So possibly the issue only occurs at idle because you’ve set the idle mix screw on the carb to be very lean, and with the #3 plug abnormally close to the strap, the path to ground is short enough that you get an arc there instead of in the chamber where the resistance is higher.

 

[Diastole]

Folks,

Phase one complete:

1) Resistance from SP Terminal to Engine Block = infinity (aka no continuity). PASSED

2) Ceramic in good condition, not cracked. see photo. PASSED

3) Spacing from Bronze Wire to Terminal (see photos)
a) Auto Lite 3076 SP = 13/32" or slightly more than 3/8"
b) Champion W16Y SP = 7/8"

4) SP gap = 0.035". PASSED

5) No carbon. Plug is clean. PASSED

6) Swapped #1 plug for #3 plug -- no aberrant spark.
(But, this was an intermittent problem initially. Only occurred about once per week, therefore not definitive.) equivocal

7) Swapped AutoLite plug for a Champion. no aberrant spark.
(But, this was an intermittent problem initially. Only occurred about once per week, therefore not definitive.) equivocal


There are photos of the Champion plugs next to the AutoLite. I agree Champion looks better and raises the bronze wires more than double the spacing from the spark advance lever. 7/8" is a significant distance for a spark to jump, and significantly more than the current 3/8".

I will try to drive the car and reproduce the situation where the aberrant spark was occurring, currently I have swapped the #1 and #3 plugs. Then I will drive it with the Champion W16Y plugs and see if I can reproduce the short with either spark plug and will update this thread.


Much Gratitude
Diastole in the Smokies


PS: I made an error the original plugs are not AutoLite TT-10 but were in fact AutoLite 3076. Sorry for any confusion.

 

[1930artdeco]

The last picture of the plug is clean because it is not firing. at least that is what it looks like to me. Is there any paint on the plug boss? I bet it is doing this going down the road and you may just not notice it.

My 2 cents worth,

Mike

 

[Ed in Maine]

This is an interesting situation, but I believe the solution is in the answers above. I wanted to point out that your picture of the spark appears to have been taken in the dark. In the past I have looked at a running engine that had a miss in the dark and found the problem very quickly. I was amazed at the arcing that was occurring on the distributor body and at the coil. Ed

 

[nkaminar]

Diastole, You will not find anything wrong with the plugs or ignition system. The spark jumps from the strap to the rod because that is the path of least resistance. The pressure inside the combustion chamber will make it harder for the spark to jump that gap than the one from the strap to the rod. I am not sure what effect the lean mixture has but it may make it harder for the spark to jump the gap at the plug.

The only solution is to make it harder for the spark to jump from the strap to the rod. That means the taller plugs, the strap bent away from the rod, and/or the rubber hose on the rod. Your choice. In my car I have done all of the above.