Technical Blown Flathead high rpm miss?

I'm using the Pertronix 8mm spiral core. I tested them for resistance and all was fine. Just the #1 wire seemed suspect, but after I made a new wire it had the same ohm value. I think it read 560 ohms while the others went up as the length increased. The old plugs were pale gray, but clean and sharp. Thanks
Only one plug gave me a reading below .05 it was a .01

 

It's the same sort of thing that causes ignition coils to fail so often in modern cars, but there it's unavoidable, it comes from how the modern engines are built and have to work.

They run lean to meet emissions standards and give good fuel economy. Lean mixtures need a higher voltage to make a spark.
The lean mixtures are harder to ignite, so the spark plug gap is wider to help that. Wider gaps increase voltage.
The compression is high, and/or it has turbo boost. High cylinder pressures increase the voltage too.

Those engines are almost trying to kill the ignition system even when they're running perfect. Add worn out plugs with wider gaps, a partially clogged fuel injector making one cylinder run leaner or even some carbon buildup increasing the compression and life gets really tough for the coils. The higher ignition output voltage also brings up the voltage seen by the control electronics, which may be more or less sensitive to that. (So when we hear about electronic replacement ignitions for old engines failing there may be a lot more to it than a bad system design, it CAN be caused by a simple bad ignition wire - that perhaps wasn't changed when the new ignition was installed, and the new stuff gets the blame.)

If you just change the bad coil and leave real cause you have only fixed the symptom, and the new coil is unlikely to last very long - another reason why some modern cars just keep eating coils, people change what's broken but fail to realize something else broke it.

Thankfully the old cars don't suffer very much from this, but that being said, it's also good to know that throwing an aftermarket ignition that can provide a gazillion volts on a misfiring engine may make the engine run fine for a while - until the unfixed original cause kill that ignition system too. You want to find the cause so you know you have fixed it, not just throw parts at it until the symptom goes away.

 

Amazing info G thank you! So where does one start to find the core problem? Lets say the plug wires ohm out fine. Does that really mean they're ok? If not, what type of testing is next? How about the plugs? They all show a similar reading, but does that mean that under extreme cylinder pressure from a blower that they're performing correctly? Would a tighter gap help fix that problem? I'm at .030 now. Plugs and wire aside, what do I check next, wire routing?
Would a ballast resistor help keep the temp of the coil down? I'm all ears! Thanks Joe

 

Excellent questions, and unfortunately now the answers are getting above my pay grade.
If the wires ohm out fine I think they are fine, at least as far as not damaging something else goes. They could still have a break in the insulation somewhere, letting the spark jump out to ground, someone has suggested spraying it all down with water mixed with something that prevents it from beading up to try to give the spark a very easy path to ground if there is a problem.

I would put my oscilloscope on the ignition system and look what hints that gives me. There's a huge amount of knowledge about interpreting ignition primary and secondary waveforms out there a**** people who do it every day, I just know enough to be dangerous.
With one scope channel showing all the sparks in order, and the second scope channel hooked to cylinder #1 (or which one is easiest to access) you can see all the sparks the ignition puts out, and knowing which one goes as cylinder #1 the rest can be connected to their respective cylinders as their order will be the engine firing order.
If cylinder #6 constantly has a considerably higher spark voltage than all others that implies the others are fine while #6 may have a break in the wire, a too wide gap on the plug, or some other issue. People who know hot to read the waveforms can get a bunch of information out of minor details, but even an amateur can see that one cylinder stands out and that's the cylinder you focus further efforts on.
If some cylinders consistantly show the signs of running lean (higher voltage, and rising up at the end of the spark duration) it would make sense to check if those cylinders are fed by the same carb/carb throat, if they are that may mean that carb needs cleaning or rejetting.
A problem that affects all cylinders may be harder, but given some time most things should be possible to figure out, and another test run using the scope would show if the situation has improved.

Not everyone has access to oscilloscopes that can perform such a job, so just standing in the back yard most people have to make do with what they have.
I suppose you could backprobe the wire connections on the distributor so you get a piece of steel wire or similar sticking out from each of them, and then just hold a grounded wire up close to them to see at what distance the spark stops going to the spark plug, and instead chooses to go to the ground wire. If they all start jumping at the same distance they're all running at about the same voltage (at idle anyway), if one or more jumps a longer distance that implies that cylinder is requiring higer voltage for some reason.

Most of my work has been on single cylinder twostroke engines, so back then something like $30 bought you a new coil, condenser and points. All new ignition system, no hesitation about if any bad parts didn't get replaced. These days with all the ****py chinese parts I'd be more hesitant to changing something that may be well working, too many new parts are bad out of the box.

 

Interesting G great read! No scope here so I'll have to try something else!