Random Intermittent Spark, clueless why

[mummyjohn]

I'm totally out of ideas now.  I put the timing light on each cylinder, and on three of them the light flashes irregularly, while on the other five it's in perfect regular sync with the engine.

The plugs are brand new and gapped.
The distributor cap & rotor are brand new.
The plug wires have got about 120 miles on them, bought earlier this year so basically new.
Ign coil is new.
Pertronix electronic ignition replacing the points is new.

So I'm officially mystified as to HOW the hell this is possible.

For what it's worth, this interesting fact: the problems are on cylinders 3, 4, and 6, which are all right next to each other in distributor order...
1-5-6-3-4-2-7-8

Do you guys have any clue what could be going on here?


This is on a 500 from an Eldorado.  It's 12.5:1 compression and running ethanol, plugs gapped at .030 as a starting point, I know lots of alcohol guys run 'em tighter.

[76eldo]

Try a different cap and rotor. One at a time.

Personally I don’t like the electronic conversions.  If it’s not the cap and rotor it could be that the wires aren’t seated correctly on the plugs. If it’s not any of that put the points and condensor back in and try that.

Brian

[fishnjim]

Look inside the cap at those #'s studs and see if they're making equal contact or not.  There should not be much white or black - indicated corrosion(resistance)/sparking - clean metal with a line across.   It's not a wiping contact per se just gets pretty close, less than the spark plug gap.   Cap could've been machined a little off center.   Could be a bad rotor, the contact bar is plastic welded in and maybe loose or off dimension.   I've had new caps and rotors be bad before.   Just because it's new doesn't mean it's right.   There's a failure rate for manufactured electrical components.   They usually don't take electrical stuff back either, if energized, which isn't right.   Part makers aren't going to test each one for cost.
Give a smear of dielectric grease on the problem plug wire contacts.  Sometimes the wire contacts are hard to seat.  They should not pull off easily.    (You didn't say what year/cap type.)   I'd swop a wire from the other side to the bad side to see if it's those wires.   Sometime they don't get crimped right.   

Could be your timing light is having issues, so check it on another vehicle.

[mummyjohn]

Give a smear of dielectric grease on the problem plug wire contacts.

Wait, what?  The other points you make seem good to check and I shall tonight, but you lost me with this one.

It's a 1971, not sure if the previous cap in there is the original or not.  What cap "types" are there?  This is "normal" as far as I know, by which I mean it's not an HEI

[MaR]

I know that this could be a long shot but on a 472 equipped CDV that I had long ago, I had a similar issue with two or three cylinders seeming having a random spark. The end result was that the intake valves on those cylinders had so much buildup on the top of the valves that they were almost completely choked off. Once they were cleaned up, it ran fine.

[TJ Hopland]

I would want to swap the wires around to make sure its not an issue with the new wires. 

What is the pickup arrangement on the Pertronix?  I just wonder if there could be a gap issue or something worn so sometimes its too far from the pickup?  If you hook your timing light to the coil wire does it appear steady or is it too fast for your light or eyes to tell? 

You running a carb or EFI?   If EFI you should be able to look at logs and see if there are any oddities in the tach signal that would maybe point you to a triggering issue.

[mummyjohn]

Swapped wires - problem stayed on the cylinders it was; it did not follow the wires.

There's something else worth noting: I had noticed, purely by ear, that taking the vacuum advance hose off  resulted in a really smooth & steady idle, and sticking it on brought back the irregular shaking.  A clear instantaneous cause-and-effect.  I finally decided to get scientific instead of anecdotal and strap the timing light on when the vac advance was removed and the motor was running smooth.  Sure enough, pure flawless timing flashes.

The Pertronix model no. ends in "LS" for lobe sensing, meaning it just bolts in and uses the existing points cam to trigger (in contrast to other models that require addition of a magnetic ring doodad).  That seemed the only part of the ignition left that could cause issues so I did research around the web figuring it's a Pertronix problem not a Cadillac problem...

...and I came across some guys saying that 90% of the issues with these ignition units are from bad grounding.  I took the distro hold-down clamp off and wirebrushed it, cleaned the part on the distro where it touches too, and the area around the bolt hole in the engine block to make a nice clean ground path.

Lo, when I fired it up again the motor ran smooth, even WITH the vac advance on.  I know there's a little wire in the distributor that straps the points plate to the distributor body.  I may have used one of the screws on the vac advance to secure this months ago, and I'm thinking that some weird extremely coincidental resonance between the systems at play where was causing the irregular "pulsing" intermittent sparking, and that the weak ground path to the block was the straw that broke the camel's back.

I hit the road and ran around for about ten miles, and it's so smooth now that at a red light I was worried I had stalled - in fact it was running fine & balanced.

[MaR]

Great to hear that it was something simple.

[Jeff Rosansky CLC #28373]

First of all, thanks for reporting back.
Glad you found it. I have been chasing a bad idle for years and I tried that as well. Wasn't my problem but glad you found it......  Now that I think of it, I checked the resistance from the negative battery to the plate, but it was not running. Maybe I will check to see if anything changes when it is running?? Worth a try.
Good for you!!
Jeff

[INTMD8]

Another thing to keep in mind is, many distributors do not change rotor phasing with mechanical advance but -do- with vacuum advance.

If your rotor phasing is far enough off to where it's firing at the edge of a cap terminal, vacuum advance can sometimes end up firing the ignition when the rotor is between terminals rather than on a terminal.

I check this by drilling a big hole in the top of an old cap.  Then you can use a timing light to check alignment.

[signart]

Your engine will fire better with a wider gap than oem when more spark is added. If oem is .035, I would open them to .040 with pertronix & their recommended coil.

[TJ Hopland]

He is running way over stock compression, alcohol, and aftermarket electronic ignition so stock is completely out the window.   I never got deep into alcohol but the little reading I did do on the subject when I was considering it there was a a lot of talk about narrower plug gaps and different heat range plugs so its definitely something that needs to be considered.   

Is this in an Eldorado or something else?   Carb or EFI?   How did you get the 12.5:1?   Is that a flat top with the early heads or does it take some shaving too?

I got E-85 all around me so the thought of building something to actually take advantage of it crosses my mind regularly.  Helped a guy build a system for a 383 in a boat but the project got stalled before it hit the water so I don't know how it came out. 

[James Landi]

I "chased my tail" most of the boating season last summer trying to get a recommended pertronix system working on a 502 Crusader Marine engine.  Tried three units...all did the same thing... backfire, rough running... went back to point and condenser... problem I created solved.  I suspect that some of the Pertronix lists of recommended engine conversions to their product is just plain wrong.     James