Definately sounds like a air leak somewhere in the engine. One thing you did not mention doing was sealing the back plate. I always use a small amount of high temp sealer on all new engine backplates as a standard proceedure. While the backplate is off check it carefully for cracks or holes. I had a new engine with an air leak that was the result of an invisable casting flaw. The only way I found it was to fill the plate with fuel and watch for leaks on the opposite side.

If all the the obvious spots for leaks have been covered the last place to look is the front bearing. I've never had Tower Hobbies engine but it seems that I've read here that your problem is a typical one that is a result of the bearing leaking. You might do a search and see if you can find out some more info. Good luck.