Ditto to what acarter said. Take out the baffle. I have done this on numerous LAs with tremendous increases in power. The last time we did it to a .25 LA, we tached it before and after and got 1600 rpm. It is really easy to do. there is a long bolt that runs through the muffler front to back. Unscrew it and separate the two parts of the muffler. The baffle is a cone shaped part inside. Take it out and reassemble. You will have to adjust the needle some because you do have less back pressure. It will be a little louder, but not bad.

What everyone else has said is true. 11X6 is too much prop. Go to a 10X6.