As a small engine specialist for the last 45 years, I would highly recommend Briggs and Stratton #19200 tach. It is called a sirometer, and measures harmonics of any rotating shaft (even electric motors), with a fine wire that you move in and out by turning the dial and vibrates in a circular pattern on the end loop when you hit RPMs as read in the window of the devise. Just place it on top of the motor on any hard mounted component (not plastic shrouds). Takes a while to develop a feel for its function, but is cheap at $24, requires no batteries. You would be surprised how accurate it is compared to a digital tach that costs 4 times the price, usually within 10 rpm or so.