Low end problem, Taurus 2.6
 

I'm having a problem with my Taurus 2.6. I accidentally ran a tank of bad gas through it. I keep one gas can in my garage that I pour the remnants of stale gas into. I grabbed the wrong can one day and used it. The motor ran like crap, of course. I realized my mistake and ran good, clean gas through it and after one tank the motor cleaned up and ran good. I've since put one gallon through it.

Ever since I ran the first clean tank, I've had a problem with the low end. If I lean it enough so that I get good transition and a smooth idle, it is almost impossible to start and keep running. Once warm, I can lean it down and it runs fine until the next flight. After today's flight, I let the plane idle for a minute and killed it with the ignition switch. I then pulled the plug and took this picture. Notice the discoloration.

Any ideas what to look for?

-Ben