In the previous article on home made Kort nozzles, the author made mention of using what he called a Spirometer… apparently a brand name of a device known more generically (as near as I can find) as a vibratach. These seem slick… a "tachometer" for measuring engine speed between 800 and 50,000 RPM, with higher accuracy at lower speeds.
You can see a picture here.
It seems to work by physically holding the device against the engine and using a "tuning fork" type principle, watching when the wire vibrates in resonance with the set RPM. Sounds a bit primitive, but several people I’ve found swear they match up very closely with various digital/electronic tools. It seems to be imported by Briggs & Straton, among others, for use with lawnmower engines and the like.
For those distrusting this low-tech approach, or just more comfortable with digital readouts, check out the Tiny-Tach. Produced by Design Technology, Inc., which produces a wide variety of small engine tools and test gear, its a very small, digital tach that can get a signal from a simple wire wrapped 3-4 turns around the spark plug.
Here’s an actual size picture:
Both of these are available inexpensively ($40 or less). Might very well be handy for those small engine generator and outboard repairs.