Also, if anyone's interested, new cartos were about 3.20 Ohms. My used (and some refilled) were 3.3 to 3.5 ohms. I would imagine it goes up the older they are? ...or just burn out (like a light bulb fillament?)
I'm not a pro on this, so I wouldn't be sure. I've heard that different batches of cartos over time have different resistances. I would guess a different composition of the coil. I'm not sure how time or use would affect it. My first guess would be that usage increases the resistance, but that doesn't feel right in my head since greater resistance should cause greater heat release and therefore cartos should 'work better' as they get broken in. My second guess would be then that the resistance would drop over time which would account for poorer performance on more heavily used cartos.
I tested all 8 of the cartos I have here at work, and 4 were 3.3, 3 were 3.0, and 1 was 2.7. The newer ones seemed to have the higher resistance, and the 2.7 was a carto that's been refilled extensively. This is hardly a scientific observation, though.