So I popped a big hole in one of my old distributor caps so I could watch the position of the rotor and I noticed right away that the firing of the rotor isn’t very stable, it’s all over the place with and without the vacuum advance.
What would cause this?

I’ve tried to load the video of what the spark looks like under the cap but the site won’t let it upload

Please explain this statement. Rotor is moving axially? Rotor is moving radially? What do you mean by "firing"? I am confused.

I knocked a huge hole in my distributor cap so I can watch the position of the rotor while the engine is running.
I can see the spark leaving the tip of the rotor and going to the distributor cap terminal, that spark was moving side to side, left to right.
Wasn’t consistently firing in the center to the distributor cap terminal.
At first I thought polarity issue but I traced my pick up coil wires back to my msd box and they are correct.
So it just has me confused as to why it wouldn’t be consistent every time it passed that cap terminal.
If this site would let me load the video you’d see exactly what I’m talking about .

i wonder if you are seeing the Multiple Spark Discharge

What ignition system do you have?

Where the spark jumps from the tip to the cap terminal is irrelevant, as long as it jumps to the correct terminal! The conditions for an arc are just slightly different on every firing, and the rotor is making wind in there as it spins at thousands of rpm, so the spark will always move around.

What's important, of course, is the rotor POSITION (phasing) relative to the terminal since that is hard-connected to the mechanical ignition timing. Is the rotor tip always in a stable position when the light fires?

krautrock is right as an MSD is multiple spark discharge under 3K rpm. A hole in the cap needs to be plugged. I recommend keeping the hole cap for tune and running a new one.

You said "popped a big hole".
Possibly damaged the cap?
How does the motor run otherwise?
Electricity, like water, follows the path of least resistance.
As said, doesn't matter where it hits as long as it's the correct cylinder/tower.

when drilling a hole in a cap, i always use a step drill instead of a big bit. when using a big bit, it almost always breaks through the cap taking a big chunk out of the inside. a step drill works best because of it's shape and cutting edge, leaving a nice hole of whatever size you want.

A 1/2" hole 2/3 of the way from center to #1 cap terminal lets you see Rotor Phasing and several holes around the side at the appropriate height helps eliminate ionized air molecules that can promote crossfire in some high powered apps (looks unsightly tho & some ain't comfortable with that & most dont bother). On your deal we need pics!

Rotor phasing is only relevant in two situations. Either at a stable and unvaried RPM or when you have all advance locked out. Otherwise the rotor to terminal relationship will change as the mechanical and/or vacuum advance occur.

What you want to do is make sure that the tip and the terminal have a good relationship thru out the advancing arc, kinda like checking rocker tip to valve stem contact during valve actuation.

You also want to check it at an RPM level that has the full mechanical advance kicked in and with vacuum advance disconnected and plugged. Otherwise see first paragraph.

Only vac adv changes Rotor Phasing.

Uh-Oh, here we go again...LOL

Joe

And then, the plane on the treadmill can't take off.
I'm outta here

The air inside the cap becomes charged. Tested this many times. Even at Schumacher Racing, arcing between two posts, jumping a gap. Needed to clear the air, words by Ace McCullough,
himself. This is why there are now vents. It helps..

http://vidmg.photobucket.com/albums/v605/sunroofgtx/Custom%20Wire%20Set/SchumacherDay026.mp4

I had the same problem. My phasing was so far out I'm surprised the spark was going to the intended post. I used the adjustable rotor to optimize the rotor tips position in relation to the intended cap terminal. I had to use almost all of the available adjustment to get my rotor to point closer to the terminal through out its mechanical advance movement.

I was checking the timing with both a dial back and an MSD timing light. You wont see the multi-sparks at idle because they happen too fast. It just looks like one big spark.

There is a problem with poor phasing. It allows a lot of heat build up due to the large gap the spark was jumping. Mine was burning up my caps and rotors. The heat build up was so bad the metal tip even melted and flew out of the rotor one time, leaving me stranded at Track Day during Spring Fling.

With my phasing issue resolved, I still had erratic rotor positioning. I had already installed the shaft collar to tighten up the free-play between the intermediate shaft and the distributor shaft drive tab. The biggest benefit of that was it dramatically reduced the wear on my drive gear on the intermediate shaft.

Frustrated, I tried locking out my distributor(its easy on a Pro-billet dizzy) and setting my timing to 35*. That completely eliminated the erratic rotor position. It is rock solid at all rpm now. I assume it was my mechanical advance springs allowing the rotor to bounce. I had been using one light and one medium spring.

In addition to the circumferential Rotor Phasing air gap distance there is also the radial rotor tip to cap terminal gap which is ideally should be brought down to ~.015" either by drilling out the rotor blade rivet & subbing in a longer piece of brass or Echlin offers a rotor with a .060" longer blade, an Echlin MO3000 for $8.xx out the door which helps. You want the gaps to be as MINIMAL as possible (without contact/interference obviously) & more gap=more sparking & smaller gap=less sparking & more gap increases the required voltage to jump said gap & is a waste of energy/stresses the coil/ign system & if the required voltage exceeds the available voltage from your choice of parts it will misfire. Also dremel the rotor tip/cap terminals being carefull not to remove hardly any brass as it takes less voltage to jump from sharp terminals to sharp terminals than it does rounded ones (from wear).

How does the engine run? How old is the distributor?

Check it with a timing light and see if that bounces around too. Try several diff spark plugs, and see how much the timing is "moving". See if it is a lot like what you see under your distributor cap.. both with or without advance.

Maybe this is way off target, but this "jumping" spark timing is what happens with Alfa Romeo distributors when the spark advance blades, pins, or slots have worn out, like over 100k miles.. They get all loose and let the timing "bounce" around excessively instead of a smooth even firing and advance curve. The car will usually run OK, the spark hits the right terminal, but the timing changes in diff situations - idle vs partial vs WOT - you lose power, and it gets hard to time it properly.

Cheers,
- Art