yes I have had this kind of problem before with one of my car engines (Force .15). And one day I took of my back plate, only to find that there was alittle rust inside.
I heard this is normal but not so good for the engine. I very helpful tip for future protection of the engine is to, after each run of flying. Let the fuel burn COMPLETELY out of the engine. So disconnect the fuel line from the engine and attach the glow igniter and try to start the engine. You will get 2 or 3 quick runs of the engine before the fuel is completely burned out of the engine.
This way the fuel wont sit in your engine and rust the internals. Ohh and if you dont have this (it's strongly recommended) that you get some after-run oil and apply 8 or 10 drops of the oil down the glow plug hole and down the carb throat.
This may seem like alot of oil being put into the engine, but the idea is to fully soak or flood the engine in after-run oil. And that the engine will be hard to turn over because there is so much oil. (Thats a very good thing). The after-run oil will keep your engine well lubricated and will keep your engine protected against corrosion and other nasties.

Good Luck!