Have a look at my machine this does the job properly you change the spring to determine how hard you want the machine to hit you change the voltage determine how quickly you want the machine hit. Each
process works independently of each other so no matter how slow you run the machine it still hits with the same power. I don't allow you to change the throw by adjustment to do this you have to change the cam(2.5mm,3.5mm and 5.0mm)

http://www.youtube.com/watch?v=9so5-dYAEfg

This is the first video uploaded I will post some more with better sound and a close up. Voltage used were 3.8v - soft spring, 4.4vsoft -medium spring ,
4.8v medium spring, 5.0v medium- hard spring and 5.8v hard spring.