Header tank fuel loss

My bobber drops an inch or so if the plane sits a while. My fuel valve is closed, too, so it’s not a leaking carb needle. I chalk it up to evaporation. Remember, the trip from all the way up to all the way down on the bobber is about 2 ½ gallons of fuel. So you’re not losing a lot. I’m not even sure we’re losing anything. But I’ve been flying it for 17 years that way!