Actually, some times ago, I was doing the same, but my tachometer was more correct with the scale to 8000rpm max, and made special for v-twin engine, and this one is up to 14000rpm, so looks like it is for 4-inline engine, and the ignition system it is very different in your engine (v-twin), and 4 cylinders. My tachometer had only 4 wires, 2 of them was positive+, the difference only one have to be connected switch light "on"+ and another to + after ignition key "on". I connected both to ignition+ d/t light is always "on" on motorcycle after ignition "on". Another wire (black) connected to ground -. you can connected it anywere to the frame. The last one (green) have to be connected to ignition coil (according to the manual). After connecting this I get the rpm on tach it is approx. 2 times less than it is have to be (according to idle, and my ear). I went to motorcycle service to fix this problem. they fixed it by connecting this green wire to the both ignition coil. after that the rpm value on tach becomes more correct, but the tach arrow was jumping a lot through the all scale during driving so the information from this tach and have it is working was minimum, and after some time I removed it and not using it any more. In case of you I am not sure that you will get any positive and acceptable result on connecting tach for 4-cylinders engine to 2-cylinders. Just check, may be there is any jumpers on tach for switching between different modes (number of cylinders), but on these cheep tachometers I am sure there is no modes for choosing it.