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Adjusting and Measuring RPM On Lawn Mower Engine

  1. Jan 31, 2012 #1

    I have a lawn mower engine and I need to read and adjust its rpm precisely. I am totally lost. I would like to ask how some of you would approach this problem.

    What I have right now is I am going to buy an Arduino circuit board controller. Then I am going to attach a tachometer onto the engine. I really don't know how I am going to change the rpm but at least this will allow me to read the rpm and record it.

    Like I said I would GREATLY appreciate any suggestions!
  2. jcsd
  3. Feb 3, 2012 #2


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    I'd like to understand why you want to do this.

    You could measure the speed in many ways. Here are two simple ways:
    - Buy a hand-held optical tachometer. Attach some reflective tape to the flywheel. The tachometer will count the number of 'flashes' per second and give you an output.
    - Buy a hand-held timing light. Attach it to the HT lead. Use it.

    The engine speed control will be more difficult. The governor on a lawnmower engine isn't very sophisticated because the actual engine speed isn't too critical. This is why I want to understand your application and tolerance on speed control.

    Why do you want to control the speed?
    Over what range of speeds do you want to control?
    Do you need to control over speed transients? If so, what are your transient tolerances?
    Do you need to control over load transients? If so, what are your transient tolerances?
    What is your tolerance of steady-state speed?
    Do you need to use the existing governor, or can you retrofit an upgraded speed control device?
  4. Feb 3, 2012 #3
    Do you have a multimeter with a frequency counter? Many have that function. Hold one lead near the spark plug wire and you will likely get enough induced voltage to get a frequency reading. I'm not sure if mowers use a lost spark or not (ie one spark per crank rotation or one every other crank rotation when it maters). It wouldn't surprise me if a simple magneto based motor fires the plug every time the crank comes around. My car has a lost spark system so I get one pulse per rev when I hold the lead next to the #1 plug wire. So a frequency of 20 hz would correspond to 1200 RPM.

    Either way, you will get a frequency that can be turned into RPM. You will have to decide if the number needs to be cut in half or not.

    BTW, an alternative to holding the lead near the plug wire would be to take a bit of wire and wrap it around the plug wire for a turn or two then connect to the meter. With my Fluke 87 this later step was unnecessary.
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