Anybody here install one before?

I have a Vapor on my 1975 CT70. Its a pretty simple deal. Hopefully you are converted to 12v?

Run the main power line fused directly to the battery. Also, if you got one with a temperature sensor that goes around the sparkplug, don't hook it up that way...it won't tell you anything useful. Instead bolt that sensor to the cylinder head in one of the mounting holes or drill and tap out (where skid plates go on newer cylinders). That way you can monitor cylinder temps which on my takegawa 124+r are consistenly about 194 degrees F.

Converted to 12 volts already.

How come the spark plus does not give good readings? Seems like it would do well there.

Because it ends up measuring the temp of the plug and not the temp of the head or cylinder. It will run in the high 300's and low 400's which tells you nothing. Put it on the cylinder...that is what you need to worry about...the heat from the cylinder, not the temp of the plug.