Check the gap on the "bad" plug. If it's correct, I guess it's bad then. Buy another plug that is the same number as the good ones....or a crossover equivalent.
Regarding continuity tests: Most meters have a section on the dial for "continuity" and they buzz. If not, then you set it to ohms. As JonG is saying: If you see any value on the display, you have continuity. 0 is a reading, or 0.0 is a reading. There is continuity. The instructions will state how your meter displays continuity.
As said, the range is important. Set it to the lowest range. Probably 200. )))) is the symbol for continuity buzz test.
If you see OL "open loop", no continuity. If you see ---- no continuity.
All this is provided you have the leads plugged into the correct meter jacks. Ignition key off may be necessary. You never check continuity with power on a circuit.
Very smart using the timing light. I learned that trick long ago when working on outboard motors.
