Petrol smell, little flat after 6K - carbs or mixture?

Pretty self explanatory really

Had reports from a mate riding behind me that my bike was giving off a bit of a smell of fuel. He said one possibility is that the carbs might need rebalanced. This may tie in with the fact that I feel the bike goes a little flat after 6Kish (not just the ECU).

So I'm wondering what the consensus might be on this? Do the carbs need balanced or does the mixture maybe just need adjusted?
Might be worth noting that the bike had a Scorpion can on it - I put standard back on which may account for something but the guy I bought the bike off (who put the can on) didn't know that much about bikes so I thought it unlikely he adjusted the mixture for the can.