Since an automobile engine spark plug fires on every other revolution of the crankshaft rather than every revolution as with most single cylinder 2 and 4 stroke engines, it will probably be necessary to multiply the reading by a factor of 2. (Even though there is a power stroke every other revolution for the 4 stroke engine, the ignition system is usually active on every revolution. However, there are a few exceptions to this rule.)
For equipment where a shaft with a known speed ratio to the engine crankshaft is available (i.e., a power take-off or trimmer head), an optical stroboscope of one form or another may be used. You will just need to paint or tape some stripes on the rotating part to put under strobe illumination:
For example, for something like a string trimmer which has a direct coupled hub, strobe disks with 2, and 3, and 4 radial lines will appear stationary under fluorescent illumination for 3,600, 2,400, and 1,800 RPM respectively.
There can be ambiguity but if you are already in the ball park, this sort of approach may be all you need.
(From: Philippe Habib (email@example.com).)
Go to a hobby shop that sells radio controlled airplanes. Plunk down $30 or so and get an optical tach. Paint 2 strips on the (crankshaft) hub of your equipment to simulate a propeller, and you're done.
(From: J. Matthew Good (firstname.lastname@example.org).)
You shouldn't need a tachometer on a trimmer. Two stroke engines in the size and power range of line trimmers can't overspeed as they don't have the power with a line head installed. Just set the carburetor up so that it 'two-fours' at wide open throttle and you should be all set. If it needed to be adjusted with a tachometer, it would have some kind of governor on it.
(From: Mowerman (email@example.com).)
B&S engines have a spring in the governor arrangement. You want to change tension on the spring to change the speed. (This is basically true of most other small engines as well but the details will differ.) The spring is attached to a metal tung in the linkage at front of motor, this is made of a tinny metal so you can easy bend this tung. To lower speed you want to lessen the tension on the spring. You can do this while the motor is running at top speed but it would be safer to do the bending while mower is not running. By pushing in tung you will lessen top speed gently as it doesn't take much to alter that speed. Often this tung gets knocked in while mowing around bushes or other protruding material and "hey presto" your engine is only idling. It is a design problem that briggs should be working on, however I love B&S engines with their simplicity and ease of maintenance.