Using a spark plug to test ignition is not a reliable test. Under compression a spark plug needs much more voltage to arc its gap! I would invest in an actual coil tester from either an auto parts store or a mower shop or similar place that handles small engines. It will look like a spark plug without a ground electrode, no threads, and an alligator clip spot welded to its side.
There are more elaborate ones, but the one I just described is reasonably priced and easy to see the spark, even in direct sunlight! :)