A tachometer measures engine ignitions then converts them to rpms.
Notice that the Harbor Freight tach is set up for 4,6, and 8 cylinder engines. For a 4 cylinder engine. it will count four ignitions as one revolution. It will probably read 4X the correct rpms on your S40.
I bought a relatively inexpensive tach which works very well on my S40. Here is a link.
http://www.jpcycles.com/product/5600076You can get this tach even cheaper on EBay. It is very simple to wire and works perfectly..
Good luck!