Header tank questions
 

Iv got a kyosho igt2 corvette with the stock .28 motor that's running really good except by the time I get down to the last 1/3 tank, she starts to lean out slowly. I can tell cause the power goes even higher and idle goes up a couple hundred (to the point where the clutch starts to engage with neutral throttle). Of the course the easy answer is to just lower the idle a little, but when I do this then she'll cut off sometimes when I come off high throttle (again this only happens about 10 mins into the run).
This brings me finally to my question. Why don't cars use header tanks (like my helis do)? Wouldn't this help keep the tuning the same from the start of tank to the end?