Hi Stu -
Originally, there was an “Idle Up” solenoid on the carburetor. What this unit would do is increase the idle speed by ~300 RPM when the Air Conditioning kicked on by pushing on the throttle linkage.
The solenoid is a cylinder about an inch in diameter that mounts next to your carburetor. Mine has been removed for many years, so I don’t have a picture.
I think what MJ is getting at is that this unit could also affect your idle performance, and might not be adjusted properly.
Your measurements show pretty clearly that you’ve got a 1.5 volt drop when your fans are on.
That lower voltage driving the ignition is almost certainly the cause of your idle performance change. My guess is that the effect I’ll be even greater at high RPMs with the fans on - the engine will start to misfire.
You need to do a wiring upgrade to get the fans on a separate circuit then your ignition.
The easiest thing to do would probably be to put in a new heavy gauge wire To realize that drives your ignition. Then you can just use the existing ignition power wire to control the relay, and this will eliminate the voltage drop.
Rocky