When you operate some of the bigger electricals in the car (including the fan)...the engine is slowed by the drag caused by the higher electrical load on the altenator.
Normally when you switch on a big electrical....a solenoid valve should open in your carburettor...this opens an extra fuel way that slightly increases the idle speed to compensate for the load.
I think your solenoid is not operating. It could be sticking...or the solenoid may not be getting its switched voltage.
I would NOT recommend adjusting your mixture to compensate if the fault is the solenoid valve.
I'm just goanna check out if theres any easy practical ways to test for this....
I'll be backkkk
