There's also risk that used harness will also have some bad wires as well. I picked one up for free that also had bad black coil-trigger wire to harvest connectors for EFI upgrade. So used harness will also need to be tested and verified good before using. In which case, you might as well test yours.
Multimeter is cheaper than harness, faster results:
HF - multimeter
How to use multimeter to measure wire resistance:
https://learn.sparkfun.com/tutorials...multimeter/all
Again, simple test to see if tach is interfering is to just unplug it. Then check for spark on #1. Depending upon how it's connected, might even be damaging to ignitor.