Your Johnson has a 12 pole stator just like the Mercs. Check your charging circuit as that can affect your tach as well as keeping your starting battery charged. The simplest way to do it is to check the voltage at the battery with the motor off. Then start it and recheck again. As you approach 2K RPM you should see an increase in voltage. If you have ever hooked up the starting battery backwards, even for a split second, your regulator/rectifier is toast. That short you have could fry it also.