All cylinders are the same after pulling the heads off and taking a look.
Ingition: Petronix I, standard coil and spark plug wires, distributor and rotor
droptop73 has a great suggestion. If all the plugs are the same, the issue is something common to all, such as the carb. It is odd the carb would all of a sudden go out of wack.
droptop73, should he check his float to make sure it is working? I never had a float go bad on an original carb, but did seem to have issues with rebuilts.
One other suggestion is the voltage at the coil. BE CAREFUL HERE and check with Pertronix, but I am pretty sure the Pertronix I is a 6 volt system. The voltage at the coil should match Pertronix. If Pertronix is 6V, you should have 6 volts at the coil(measure + side of coil to ground with ignition key on). If you are getting less then what Pertronix says, that might be your issue. If nothing else, it is something easy to check.
Did you really pull off the heads? If so, what do your exhaust valves look like? Maybe you could post a photo.
Did you put the Pertronix Ignitor I in yourself?
How old is it?
This is probably not a factor with your current issue, however, for what it is worth:
1. My understanding is the Ignitor II is more reliable and has better dwell adjustment over the RPM range then the I. I never had the I, but I have about 60K miles on a pair of Ignitor II's. Superb as far as I can tell.
2. droptop73 can confirm/deny this, but I think we would agree the Pertronix I, II and III are NOT high energy ignition systems. They are simply LOW energy electronic replacements for mechanical points. A future modification you might consider(like if/when the Pertronix module malfunctions) would be to go to a Duraspark distributor with an MSD or HEI module(I venture to guess droptop and I would disagree on the module).
Finally, when you do put things back together, do check out the spark. You'll know and hear a good spark when you get one.