Coil arcing...351 running like #@$%!

[MartyG]

I cant figure out the issue. When I crank it, it's VERY slow, almost like battery is low...then it kicks over. When it's hot, it's worse. Battery is fine.
It's running very rough...some backfires. Rebuilt the carb (4bbl Holley) and thought it's dialed in.
I checked it with a timing light and it's dead on. Pulled all the spark wires to see if a cylinder is missing and all are good.
I have an MSD unit and I'm seeing arcing from the (MSD) coil from the pos. terminal to the center post.
Coil has no cracks in it. Swapped out a new coil wire...same issue.
There's no vacuum from my vacuum advance but that can't be all of it. Checked for other vacuum probs and can't find any.
What...is...the DEAL?! Any ideas from you enlightened gents are much appreciated.

 

[bronconut73]

I used to have my MSD High Vibration coil mounted to the front of my head. It seemed like it got kind of hot there and I would get an occassionsl similar issue.
I moved the coil over to the fender. Just had to make a slightly longer coil wire.

Maybe because those high vibration coils are set in epoxy and not oil they cannot take the heat as well.....

 

[tortoiseracer]

Sounds like a bad ground from your body to engine block. Make sure the grounding strap is clean and connected or add a ground jump from body to engine block. Can try it out by using a jumper cable. Run it from the ground on the battery to the block.
Chris

 

[pcf_mark]

Check that the boot on the coil is really tight. If so then check your spark plug wires. One or more of them is likely disconnected or burned. When the arc can't jump the gap across the plug or to some other ground it arcs right out the top of the coil.

It is probably running poorly because it is limping around on 5 or 6 cylinders.

An interesting way to track it down is to put the timing light on each wire one at a time working your way around the engine. Just aim the light at the palm of your hand. It should be very steady. Try each one and you will find the sketchy one pretty quickly because the timing light will flicker. Then start looking at the cap, wire, plug for the cylinder. If you get several that flicker start with the cap.

 

[roundhouse]

Cranking over very slow is a sign of bad cables or a overly hot starter from the headers

Also your hood brace is probably rubbing the dizzy
Put the mustang rubber boot over the dizzy


Sent from my iPhone using Tapatalk

 

[DirtDonk]

The high resistance to spark could be coming from inside the distributor too. If your advance mechanism has failed somehow, your rotor may no longer be phased correctly (aimed at the right post at the right time) and your first spark inside the distributor cap might be having trouble getting to the right post.

Your MSD box (a 6al presumably?) is a CDI (capacitive discharge ignition) sends a 300 volt signal to the coil's positive terminal just miliseconds before it's supposed to fire a cylinder. There is no longer a simple steady 12v wire there.
It's possible I suppose for that 300v jolt to be jumping the gap, but the more likely culprit is the high-voltage spark from the HT lead like was said.

Good luck. Finicky little things when something starts not liking it's life.
And even though you didn't change anything prior (or did you?), I would definitely clean and re-tighten the battery cables, make sure there is a ground wire from the battery to the body (not just to the engine) and then the engine to the body at the firewall.
Even though this might not have anything to do with today's issue, the stock grounding bond between panels deteriorates every year until funny electrical gremlins start popping up.

Paul