With this type of inline spark tester
you plug one end into the spark plug cap and the other end fits over the plug, like the original spark plug cap, so you can start the trimmer up with the spark tester plugged in. Then you watch the coil in the tester. If the tester coil begins to lose spark at the same time the trimmer starts to die, you know your coil is bad.
The tester is a whole lot cheaper to buy than the coil and you end up with a good tool even if the coil proves to be OK.
As for the US carb working - for 1/5th the price I'd take my chances, but test the coil first.