The Charcoal Filter is apart of the emissions system. It is designed to recover the gas fumes from within the tank to burn them up in the engine. If the charcoal inside the filter is no good, it is unable to absorb the gasoline vapors. When gasoline vapors escape into the air, they only disappear for a while. Aside from creating smog, when it gets cold in the morning, the vapors will condinsate back into a liquid form onto plants the exact same way morning dew does. As I belive it, a common symptom of a charcoal canister needing replacement is the "Hot Start" problem. To add something else to this, my question is: Is a replacement of the entire assembly nessisary, or can you refill the canister. And if so, will regular aquarium charcoal do the job? -Robert