fire jumping plug wires

[390owner]

On my 390 I bought a hei dist a few years ago. Here in the past few months I have had a miss on hard pull at low rpms. I changed the plugs, wires, dist cap, coil and ran a new wire from the dist to the fuse block. The other night I  opened the hood with it running after dark and it looked like my plug wires where see through. There was blue fire going everywhere around the plug wires. What is causing this? It was doing the same thing before I changed all the parts mentioned above. I also have several grounds I checked under the hood to make sure they were clean and tight. The hold down on the distributor is also clean. I am out of ideas

[galaxiex]

I had that happen on a 390 when I had one of those aftermarket clear dist caps. (remember those?)
I changed the cap to a good quality stock type cap and the issue went away.

You already changed the cap and more so IDK....

I also has a 65 Mustang 289 that would miss on hard acceleration.
Turned out to be a cracked radiator top tank.

Normal driving was ok, but when I hit it, the cooling system pressure would go up,
and the crack was on the engine side of the rad top tank, right in line with the dist.
It would spray a very fine mist of coolant right at the dist cap.

When I would slow down and stop and lift the hood, the heat had evaporated the water/coolant and the cap and rad was dry.

Took me a long time to find the problem, until one day I revved the engine hard with the hood open.
Then I saw the spray from the cracked rad tank.

[e philpott]

Cheap wires or you don't have them plugged all the way on to the spark plug, spark finds the easiest way to go

[jayb]

Or, not plugged all the way onto the distributor cap, or spark plug gap too large, or, in this day and age, even brand new plugs can be defective...

[390owner]

I was using acell plug wires then I changed to ford racing wires. I am using motorcraft plugs. I also put dielectric grease in the plug wires then boots. They are pushed down all the way on both ends

[jayb]

If it's missing and you are seeing all the arcing under the hood then it seems that the plugs aren't firing all the time under load.  One test might be to reduce the plug gap to .020" or so, see if that helps.  Water under the hood will cause that problem as well, we used to spray WD-40 on the wires and distributor cap, and even under the distributor cap, to drive away any moisture that could cause the problem.  I assume you are using resistor plugs and resistor plug wires.  You can check the electric resistance of the wires with an Ohmeter, usually I see them in the 5K to 12K range. 

If you are seeing the arcing primarily on one wire, figure out which cylinder that is and pull the valve cover on that side to look for a broken valve spring, bent pushrod, or some other issue.  Not likely that this is the problem if the engine is quiet at lower engine speeds, but something to consider.  Hope some of this helps - Jay

[MrRupp]

I would check for a crack in the porcelain on the spark plugs.

[GerryP]

Was it a cool, damp night with the temperature near the dew point?  It's not unusual to have a corona around the plug wires or any high tension wire under the right climatic conditions.  If you have a condition where the resistance is very high in the high tension side, it causes an increase in voltage and that can cause the corona.  As others have stated, with all new components, you need to tighten up the gaps to reduce the juice it takes to jump the gap.  If it persists, you need to measure resistance on the plug wires.  Normally 2kOhm per foot of plug wire or less for a good wire.

[390owner]

Ok I will double check my plug gaps and maybe check the wire resistance Thanks