I always run the blower for four minutes before I start the engine. The purpose is to clean out any gasoline vapors. If you have a gas leak or vapors are escaping from your engine, carbuerator or old hoses, the vapors build up in the engine room and the spark from the starter can cause an expolsion. It happens at least once or twice a season in the country.
Even better is to sniff the engine room before starting or if that's too tough sniff the blower exhaust.
I don't run the blowers after the engine starts, even at idle speed. The engine is sucking 5.7 liters of air every two revolutions, any vapors will be consummed. Can't hurt to leave the blowers, on just wears them out faster.
When fueling you have to be extra careful, because now you have vapor generated by the fuel nozzle. I turn the blowers off during pumping, otherwise they might suck in vapors from the nozzle. After the pumping is complete, I turn on the blower while I go in and pay, at least 4 minutes.
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