Leaf Blowers

I bought a 350BT on November 12, 2012 through Amazon. It ran great the first three times I used it. The next time I tried to start it, it was very difficult to start and when running it was rough and kept going dead. Eventually I figured out that it would only run with the choke about halfway closed. It ran OK that way, but would not reach full power, and was very sensitive to choke position (still would not run if I opened the choke all the way).

The plug looks fine except for being a little dark. I'm using the correct 50:1 gas mixture with good gas and partial synthetic oil. I have many small engine devices, including several Husqvarna products. I don't feel I've done anything wrong. What could be the problem, and what should I do?