I had the exact same problem when it got colder too. I finally said screw it and took it home and tore it apart. Found a lot of stuff (grass, dirt, etc...) in the carb. I put it together and fired it up. I also put the muffler baffle BACK IN! Used the pinch test to set it up and she now can idle for hours. I dont think she likes the loss of back pressure when it gets cold. I would at least clean the carb and put the baffle back in. I'm guessing you have it out.