My 65 425 has developed a bit of a stumble right off idle, and seems to idle sort of rough. I checked and regapped all the plugs and noticed that a couple of them seemed pretty dark. After starting it back up, I put the timing light probe on each of the plug wires just to see how it would register. I got good consistent flashes on all but wire #5, which would not register at all. I swapped wires between #5 and #7 and the problem stayed with #5, but became sort of an intermitent flash, like it was not catching every firing command. New Pertronix II and coil. New cap, rotor & wires. The wires are sort of el cheapo, @ 7mm. What do you think might be casusing this? Do I have a bad Pertronix out of the box?

 

Swap plugs between 5 and 7 and see if that makes a difference.

 

I was just going to type the same as Joe's suggestion but was beaten to it. My bet is on spark plug(s). I would never change one spark plug - I'd change them all.

 

Bad plug, wire or carbon tracking inside the cap. Swap things around one at a time as suggested to see which part it follows.

Good luck!!!

 

I swapped 2 plugs, and the problem followed the plugs. New set of NGK's on order. Thanks!

 

Just to follow up, a new set of plugs solved the problem, all cylinders firing! Yay!

 

Randy - Over the many pieces of equipment over the many years I've changed out spark plugs it's my impression you can't tell a bad spark plug from a good spark plug by looks alone. Sometimes you might get lucky and actually see a broken electrode (not very common), maybe a rusted plug, rusted electrode, maybe a crushed electrode or whatever. I am of the opinion the insulator material gets cracks in it (which you cannot see) and or the resistor embedded inside the spark plug breaks down. When the insulator material cracks and/or develops hot spots, sometimes you can see a hot spot near the tip where the center electrode is contained & protrudes, but you may not see a hot spot. In any case, if the insulator material breaks down, develops cracks, etc. you lose electrical energy. Aluminum oxide is the best insulator material, but I'm not certain all spark plug manufacturers use Aluminum oxide & if they do, I'm not so certain they use quality Aluminum oxide. It's all fine & well to broadcast all the supposed benefits of Iridium, Platinum and the like but honestly if the Aluminum oxide (ceramic) insulator develops cracks, pits, etc. you begin losing tons of electrical energy which is supposed to be delivered to the electrodes; and, when it's not - you have a failed spark plug which you can't actually visualize since the insulator material is contained w/in the spark plug itself. Just my opinion.

 

It's sort of crazy, there ended up being 2 plugs that acted eratic. Both were fairly new, and both looked fine, except for the carbon buildup on the tip from not firing properly.

 

I'm always amazed with a spark plug that does not spark. It's not like there's a whole lotta parts in there that can break.

 

I've had many AC and Champion plugs that were bad right out of the box.