If you need to know how many rotations per minute (RPM) your lawn mower engine is making, there are a few different ways that you can do this. The most common and straightforward method is to use a digital multimeter.
To check the RPM of your lawn mower engine using a digital multimeter, first make sure that the engine is turned off and cooled down. Once it’s safe to work on, remove the spark plug wire from the spark plug so that there is no danger of the engine accidentally starting while you’re working on it.
Next, locate the crankshaft pulley at the front of the engine. This is usually a large wheel with grooves in it that the serpentine belt wraps around. There will be a small hole in the center of the pulley – this is where you will attach your multimeter probes.
With one probe attached to each side of this hole, set your multimeter to its “Hz” or “Frequency” setting and touch each probe lightly to their respective sides of the hole. You should see a reading on your multimeter display – this will be how many hertz (Hz) or cycles per second (CPS) your engine is currently running at. To convert CPS into RPM, simply multiply by 60: for example, if your CPS reading is 4 Hz, then your RPM would be 240 (4 x 60).
Another way to measure RPM without using any tools is by listening to how fast the engine sound is going. This isn’t as accurate as using a digital multimeter but can give you a ballpark idea of what range your RPMs are currently in. For example, if it sounds like the engine speed has doubled compared to when it was idling, then you can assume that RPM has increased by approximately 1000 rpm.