I'm also very intersted for an answer to the original question.

I just bought a Tower Hobbies tachometer, and no mater where I place it on my Homelite 25cc, I get readings from 6000 to 9000 rpm at full throttle, but it never stabilizes, it keeps going up or down.

Should I place it in front of the prop instead? and where should it be, at the base, middle or tip of the prop?

Thanking all in advance.