Skynard,
If you have Tower Trainer you are right, there isn't any room to put foam in. I learned on the same trainer (still have it today, 9 years after I learned) and I thought the same thing, that it needed foam in there. While it would be better to have it surrounded with foam, it will be just fine in this plane. Don't worry about the foam here.

It does sound like you are having motor problems. I have had one Tower engine and I found a great use of it. I cleaned it up and it makes a great paper weight on my desk. I would replace the motor and try again.

Hope this helps