Rotor changes timing, 123 running fine, but? Options

[mgphoto]

So my engine has been really running fine except for a miss that seemed to come on at about 3200 rpms. I'm running the 123 D Jet version #B switch and no vacuum connection to the dizzy. I've pulled the timing back to 26* and I'm using regular gas with an MMT additive to control ping. This setup has been working very well as I've been testing different brands of the additive for effectiveness.
The miss I've attributed to cap, rotor and wires. I replaced the cap and rotor and before replacing the wires I took it for a spin.
The miss seemed to be gone but the ping I had tuned out was back?
I hadn't moved the dizzy, I tightened the clips holding the cap but no other change.
So I pulled the cap and took out the rotor to see what might have changed.
The new cap was identical to the one that came with the dizzy, but the rotor not so much.



I suppose that could be a 2* difference in the rotor pickups?
I put in a rev limiting 5850 rotor I had in my stash and now no miss and no ping!
If there's an issue it will find me...

[Rand]

There's a difference between a "longer opportunity" for spark vs when the spark actually happens. The points opening (or electronic trigger) is the timing. That has to be precise.

[clapeza]

There's a difference between a "longer opportunity" for spark vs when the spark actually happens. The points opening (or electronic trigger) is the timing. That has to be precise.

Exactly. It's the triggering inside the distributor that determines the firing. What happens inside the cap is trivial. There is no direct electrical contact closure inside the cap between the rotor tip and the spark plug wire connection on the rim, only the center pin of the cap and the center contact of the rotor.

Try this: disconnect the spark plug wires from the cap and put your multitester leads in the center and one of the spark plug wire connectors on the distributor cap. Set for continuity. Now turn the engine manually and look for continuity. It won't happen.

The reason you get spark to pass from the rotor tip to the cap leads is that the electrical charge jumps the gap between them. Remember, we're talking upwards of 50,000 volts here, plenty to jump a fraction of an inch. It jumps because the charge is looking for ground, which happens to be available via the spark plug wire to spark plug to head route.

When you're setting the timing, you're doing very little inside the cap. It's what's happening inside the distributor - when you turn the body of the distributor, changing the relationship of the distributor drive gear to the firing point in the rotation. That firing point being under the control of when the points open or the electrical circuity says so, in the case of the 123Ign.