I have the same Arrow with the Evolution 46 (actually a .45). The plane flies very nicely when the engine works, but I have had tons of trouble with the Evolution. Twice the muffler has come apart, and as you have discovered, it's virtually impossibly to get the muffler to stay together. When I posted a similar question a few weeks back, one suggestion was to throw away the muffler and replace it with the Tower hobbies 46 muffler assembly. I have not had a chance to try this out yet. I sent my engine back to Horizon hobby for repair since it also started making a lot of noise both at idel and at WOT.
I did order the muffler from Tower Hobbies though, and will try this when I get the engine back. Costs $14.95.

I also found that the glow-plug that came with the engine didn't work very well. Seems like any other standard plug works better and as 2slow2matter suggested, you need to make sure the low speed needle is adjusted right.
I didn't have any problems with tank pressure and the fuel supply when I removed the baffel inside.

Good luck,
tycho