Thanks Water Warrior. The previous owner didn't tune the bike, and mentioned he often used the choke to keep it idling, never touched the idle screw on the carb.
I got it for a deal, but the sound does not inspire confidence when riding. You don't think this sound could be an early warning of a major issue? I understand gz250's sound like sewing machines, but this has a clear metal on metal plinking each rotation, all temperatures, gears, and acceleration.
In short, is there anyways to reduce the noise rather than live with it? I already used a bit of lucas oil stabilizer and this did make the tap a little less distinct.