Batz,
I also have an Autometer Tach that only reads half the RPM's. I have the loops cut as well and wired correctly. I contacted Autometer tech department. They told me that I had to have the tach recalibrated to work with the pulse rate of the Yamaha ignition system. Basically, we have purchased automotive callibrated tach's and connected them to a motorcylce engine. They did tell me that it was no big deal to recalibrate, just would be tied up for a month or so. I decided to wait until the season is over and then send it in. You can contact them through their website if you need to.
KB