Tachometer wiring?

Hi, I have a set of clocks from a Shineray GY125 (Speedo and tachometer) and I am having a problem with some of the wiring on the tachometer. There are 3 wires from the back of the tacho but I am stumped as to which wire goes to where. There is 1 black, 1 green and 1 yellow wire with a black tracer. I was told the black is earth, the green goes to the coil and the yellow and black goes to the ignition i.e. switched live. The problem I have is that the bike I am wiring it to is a Gilera Coguar and I am not quite sure where the connections are on the Gilera as the wiring colour scheme is no doubt different to that of the Shineray. Any help will be greatly appreciated. Thanks in advance, John.