Dave had just installed the JPI RPM sensor on magneto on his O-320-H2AD. During the run-up, left mag: 350+ RPM drop.
He canceled the flight and checked out the data.
Minutes later, he had the answer.
See that orange line? That's cylinder #2. Every time Dave switches to left mag, it goes cold. Textbook single-plug failure.
Here's what healthy looks like—all four EGTs rise together on each mag. On Dave's engine, #2 drops like a rock.
One cleaning. Next run-up: perfect.
The Takeaway
Without the data, this could've have meant at best, pulling all the plugs, or at worst, a parts-swapping odyssey — harness, impulse coupling, mag bench test. Instead, one trace pointed straight to the problem.
The sensor paid for itself before the wheels ever left the ground.