The 18x6w might not be too much prop for the engine, it depends on the numbers, but a 18x6 would be ideal. Arobatx is correct, as an aftermarket muffler is much lighter, gives much better performance and though louder might be the answer to your mysterious engine silence, as I think that the very quiet nature of your stock muffler is killing the sound of the engine's exhaust when in a high speed dive at a low throttle setting.

Karol