I bet it's been covered before. I've got 2 different planes, both start out with a decent "4-stroke" run and get a nice 2-stroke break during maneuvers. About halfway through the tank they lean out and I'm flying too fast to make good things happen. I am running LA 46's, 10% nitro with synthetic (17%?) and 4 ounces per gallon of Sig castor oil. One uses a tongue muffler, the other a standard muffler. They both use uniflow tanks (4 ozs.) and no muffler pressure. I have tried raising the tanks as much as 3/8" above center with no improvement. The props have been 12-6. Could this be the famous back plate leak I've been hearing about? But only after 1/2 tank is consumed? Any other ideas would be greatly appreciated.

 Yes, you are right about it being discussed many times.  You will get a lot of different answers many of which will work depending on ceratain condition.  I've run these engines as much as many other people but never with a 12x 6 prop.  That would be the first thing I would change.  These engines aren't designed to "break" like a lot us of would like.
      These types of problems are related to many things.  Air leaks in the fuel system or a leaky plastic back plate can be problematic as well as tank height.  If you are using the remote needle assembly is another suspect.  If all this stuff is OK I would bag the prop and go with something with less load.  Heat is another thing that will cause these symtons.
      Here is the preferred set-up around these parts.

    .280 venturi, .156 spraybar, Aero products from Randy Smith or Hobby Fastener's Super Tigre clone from RSM and no plastic backplate.
    10/22 Powermaster fuel.  I personally have had good luck with Wildcat 10/18 and Powermaster 10/18.
    Thunderbolt RC idlebar glow plug, hot.
    Uniflo tank on muffler pressure, aproximatly 4.5 to 5.0 ounces.  More or less depending on weather.
    Most importantly the prop.  APC 12.25 x 3.75 at 9800 rpm's.  APC 11.5 x 4 is second choice.
    This a known combinations that has worked well for a lot of people.

    I think overloading the engine is the biggest problem you are having right now.  Others will chime in here and they are going to want to know airplane weight, design, line length, lap time and many other factors that will affect your run.