Should you empty the tank right away after race your nitro? Or can you leave it in there for a week or so?

Never leave the fuel in your nitro for a week or so. You should empty the tank as soon as possible after your days run. You should also make sure that you burn off any extra fuel left in the engine. Vital engine components will rust out causing them to break and dramatically reduce engine life. You should also use after run oil after each use to keep the engine lubricated between runs.