Wayne, I have a gms .40 and am having the exact same problems! The only difference is that I have a pitts muffler on mine. I found today that Tower recommended the wrong muffler for this motor. If you put your finger over the muffler exhaust partially covering the outlet it would rev up fine and not stall out. So I called them. The muffler that I have does not give enough pressure back to the tank to pressurize the tank due to it having 7/16" tubing for the exhaust as the correct muffler has 3/8" tubing for the exhaust. They told me I could return the muffler but it has already had the tubes cut to fit my corsair. So I would have had to buy a new/correct size muffler. WRONG! Not for another 40 bucks! I am going to slip some copper tubing inside the too large of exhaust tubes and cut the exhaust down a bit. This motor was starving for back pressure. Yours sounds like the muffler you are using is causing yours to starve for back pressure also. You might want to try what I said about partially closing off the exhaust with your finger as someone holds the plane and you run it up to full throttle.

Rick