What causes a distributor rotor center tab to burn through?

[dmjlambert]

OK here's what I have: 1969 CST/10 with rebuilt 350 engine and TH400 transmission. The distributor is a GM HEI from 76-80 and I'm supplying it full voltage. The Quadrajet is one from National Carburetors and 1985-86 newer design with electric choke. The engine has worked fine for years and is my daily driver. I use it about 5000 miles per year.

Late 2024 it started running bad so I decide to just give it a tune up which was new spark plugs gapped to about .040, new AC Delco cap and rotor, and I sent the carburetor to National Carburetor to have them fix a couple of problems such as Quadrajet bog due to probably accelerator pump going bad and it was not holding fuel in the bowl more than 1 day, and the electric choke was finicky. So, they got it going and sent it back to me.

I put it all back together and it had idle speed not steady and general running bad. Base timing is at 7 BTDC which is about right for this engine with the vacuum can I have (it has a lot of vacuum advance range with that vacuum advance can). I've only driven a few miles since it doesn't run well.

I found #6 spark plug wire had fallen off, apparently I didn't click it onto the distributor cap sufficiently, and I put it back on and it ran a little bit better. I found probably the more significant problem, the vacuum advance can went bad and would not hold a vacuum nor move the advance. The can being bad was a vacuum leak (through the bad vacuum advance diaphragm). So now I have a new vacuum can that I installed. I have not yet set the timing, but will do so when I get it all back together. It will probably like a little more advance because I got a new vacuum can with less range.

When putting it in I noticed the rotor had carbon buildup and the center tab was burned through. What the heck? Would going just a few miles with 1 spark plug wire loose cause something like that? Or the vacuum advance can being bad cause that? Is my assembly order wrong or missing parts or do I have a bad coil, or what? In the cap I put the rotor contact button, then the spring with fat side down on the button, then the rubber gasket, then the coil. The stuff is in the order shown in the picture. Is that right and am I missing parts or is that it? Are there any tests to do before I get a new rotor and put it back together?