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The cause might be related to the torque required to drive the alternator when the fans kick on, the alternator can put a significant load on the engine even if it isn't keeping with load demand at that RPM. I would not change initial advance to solve the problem because you should not alter the curve assuming it's right in the first place. In the 70's they used to use a "throttle kicker" on engines to help offset A/c loads IE speed up the idle when the a/c was turned on. If power was cut to the a/c the engine would drop back to normal idle speed. These devices could be adjusted for idle speed in both on and off conditions and mounted to the carb at the idle stop. Perhaps you could use one from a salvage yard. I have a similar problem to yours but it is related to low voltage when the fan comes on. My problem is the voltage drops to a point where the fuel injectors don't function properly at 11.5 volts and the engine won't idle. My alternator is only a 50 amp Kubuto tractor unit that can't match the load at 2000 rpm.. Since you have carbs I would bet it is alternator drive load causing the problem.
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