Thanks all!! Here's the tach I'm trying to install:
Motorcycle Universal Analog Tachometer Gauge LED Backlight Speedometer 0-13000 RPM （Black）
Wiring instructions are:
The tachometer has two line groups, which are 2 and 3 respectively. 2 strips are the backlight line of the instrument, regardless of the positive and negative poles, and the positive and negative lines of the illumination line. Three black wires are connected to the positive pole of the power supply, the electric gate is locked to the power output line, and the green wire is connected to the negative pole of the power supply. Yellow is the speed signal line
Looking at more diagrams of similar tachometers it appears it's expecting a signal line. The coil I'm using has the orange line from the cdi, the ground, and the spark plug wire.