It could possibly be a lifter losing it's prime at low rpm (low
oil pressure) and as the rpms come up, so does the
oil pressure, getting enough
oil to the lifter to make it work properly.........
try getting the car up to operating temp, say a 5 to 10 mile drive and then while it's still hot, change the oil and filter.....sitting that long could have caused things to "gum up"....
Next I would definetly take the carb apart and go thru it or have someone qualified do it for you, carbs will definetly gum up and gaskets will dry up sitting that long........
David