distributor cap wear related ques

SteveO_71 · May 11, 2017

I have a 71 Mach 1 with 351c 4v and Pertronix II ignition and coil. I was checking the dist cap and noticed arcing marks that I havent seen before.

They seem to be occurring to the side of the terminal in the direction of rotor rotation. Actually disintegrated a little of the distributor cap material. In the past I've noticed arcing right on the terminal. Is this normal or something else happening??

View attachment 38435

Don C · May 11, 2017

How much timing advance are you running?

The location of the cap locating notch may be slightly off, causing the terminals to be slightly rotated.

turtle5353 · May 11, 2017

Rotor phasing could be slightly off. I got an adjustable rotor for my msd dizzy. Works pretty slick.

SteveO_71 · May 11, 2017

Even if the cap was rotated a little, wouldnt the high voltage arc still occur when the rotor was at the terminal as opposed to the side of the terminal? Its almost like the rotor is finding a ground at the side and firing the plug as opposed to waiting until its lined up with the terminal. Or am I understanding this incorrectly.

Thanks!

turtle5353 · May 11, 2017

Even if the cap was rotated a little, wouldnt the high voltage arc still occur when the rotor was at the terminal as opposed to the side of the terminal? Its almost like the rotor is finding a ground at the side and firing the plug as opposed to waiting until its lined up with the terminal. Or am I understanding this incorrectly.

Thanks!

The terminals on the cap do not tell it when to fire. It just receives the voltage. The reluctor wheel under the rotor and the magnetic pickup are what tells it when to fire. If anything is off a little bit it can cause the spark to try and jump to the terminal too early or too late. Neither of which is good. One way to check it requires sacrificing a cap. You cut a hole in the cap on top or side so you can see the terminal inside and the tip of the rotor. Paint the tip of the rotor with white marker paint. Fire up the car, and hook up a timing light to the plug wire that has the terminal hole cut. you will be able to view when the spark jumps from the rotor to the terminal. Ideally you want it to spark just as it gets to the terminal. Least amount of air gap to travel across. With an MSD adjustable rotor you can loosen the screws and rotate the rotor till its firing in the position you like. With a stock type of pertronix, you would have to slot the screw mounting holes to be able to adjust it. There are lots of youtube videos and articles online explaining the theory of phasing way better than I can. My explanation was pretty rough. Hope I didn't confuse you too much!!

Also a non-vent cap can build up ozone inside and cause early or cross fires. FYI.

http://www.onallcylinders.com/2017/01/20/ask-away-jeff-smith-rotor-phasing-relates-efi/