My best guess is that the battery is getting a bit tired or something is amiss with the alternator / regulator. A healthy battery with a decent charge should be able to run the fans for a while without that much voltage drop even without the help of the alternator.
What is the battery voltage at idle both with and without fans and also at around 1500RPM? If the alternator isn't providing 13.5 to 14 Volts to the battery on a fast idle you might not be getting enough charge into the battery.
I just re-read the post and now not sure I fully understand. If the voltage on the coil is lower it may result in a weaker spark but that would tend to cause misfiring. If the engine is running smoothly but 200RPM lower the coil voltage may not be an issue. It's just the load on the alternator slowing the engine.
Briefly shorting across the ballast resistor will tell you if the voltage drop is your issue but your cunning plan would probably toast the coil if used for more than a few minutes.
Last edited by David C; Yesterday at 07:54 PM.
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