Chip Rathbun offers this useful gizmo to help tune your motors:
This is a great little tach. Works on outboard motors too. Just mark the flywheel with two black strips 180 degrees apart. Point the tach at the flywheel and take the RPM reading.
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXPT31&P=ML
Mark Lindhorst adds:
FYI… “tiny tach” at http://www.tinytach.com/ works great too. About the same cost, you wrap a wire around the spark plug wire and a little device gives you a digital read out of rpms. The latest model is adjustable for many different applications as well.
Mark