Rewired an old home made coil / spark plug tester today and tried it out. Only had a AA battery in the house, but got not only one spark .. but two!
That is not considered a good thing. Most probably due to the lack of 0.005 clearance on the top contact of the coil......
More in the MTFCA electrical booklet.
I just grabbed an old spark plug out of a junk box. I never thought to adjust the gap, but it's interesting to see.
Looks like a mostly fouled plug. Try another one.
To properly visualize coil output double sparking you need ta spark gap where you can view space and time relationship.
Your test fixture, just like a buzz box coil tester, only provides one dimension. Space and no time.
Ron the Coilman