|Testing the Tachometer|
RPM = count / time * 60 seconds * 2 revs per firing / 2 cylinders
For my measurement it was:
RPM = 86 / 2.62 * 60 * 2 / 2
RPM = 1969 = -2% from tach
-2% is as good as perfect since I couldn't very well expect to set the engine RPM more precisely than that anyway. One cylinder fires once every 2 revolutions (4 stroke engine). With 2 cylinders you get a firing every revolution. So the number of firings per minute is the RPM for a 2 cylinder engine. For a three cylinder it would be 2/3 (and so on). I also recorded at 1200 and 1500 RPM. It was harder to read the firing on the 1200 RPM recording because it was more masked by ambient noise. But it did work.
MP3 recording of engine at 2000 rpm...