One thing I have had problems with on the pro 15 is air leaks, especially on the backplate. How exactly did you check for air leaks?

Why not pull the backplate and clean things up there, seal it good with a sealant. Also pull the carb and clean it very well, replace the o rings ($2.00 for O-ring kit), put it back on and seal it with sealant also.

There are a couple easy and inexpensive things to do right there, and quite often they are the problem. I would also make sure the gas tank has no leaks on the seal. Pinch the line going to the carb and blow air into the line that connects to the exhaust, it should hold a little pressure.