[Steve Jones]

Michael,
I'm at work and don't have any of my notes, so bare with me and take these numbers for what their worth. The O2 sensor output voltage varies depending on the type of throttle & load input, i.e. accelerating, cruise, decelerating, etc. and therefore, which carb circuits are active. As I recall, the voltage would rarely go below .8 V and only under thottle off deceleration. If memory serves me correctly (and it rarely does), a 14:1 air/fuel ratio should produce about 7.5 V. Anything above that is richer and below is leaner. I have settled on #71 primary jets and #80 secondary. I've tried all sizes down to #65's in the primaries (to the point where the car developed a "lean stumble" on transition from cruise to accel) and the O2 sensor still indicates rich most of the time.
I found the same thing as you with respect to the thermostat. I originally tried using a 160° and found that this engine needs to be at 180° to run good. Been using a 180° ever since.
My problem isn't that I'm fouling plugs under normal driving conditions. When I have a problem is when I have to start & run the cold engine for a short period of time such as loading it in the trailer or just firing it up during the winter to keep everything lubricated. If I don't run it for a good 15-30 minutes and get it totally up to operating temperature, the NEXT time I start it, one or two plugs will be fouled. I'd sure like to be able to load it on the trailer without worrying if any plugs will be fouled when I un-load it. If I'm loading it at 6AM the neighbors don't want to hear it running for 15-30 minutes!!