The best time to do this is when you are changing the engine oil. Remove the air filter from the carbs, being careful not to tip it as it is full of oil. Unclip the top of the filter and empty the oil from the bottom half into your oil drain pan. Clean out the bottom of the filter as it will be full of sludge. Put some clean oil in the filter up to the red line. Clean any sludge from the rubber breather pipe that runs from the oil filler to the air filter. Put everything back together and put back on the car.