Carbs comments above are a place to look – in addition some other thoughts
Is this issue occuring riding in the winter cool days – these carbs did suffer from icing issues and later models such as ZX1000b1 and 1000RX has carb heaters fitted which was a loop of warm water from the cooling system. This would manifest itself in engine starting to run rougher after a number of miles and needing to use more throttle on accelerations and smelling of fuel when stopping.
Also pull out your plugs and see what mixture is like – if black and sooty check your air filter, air filters on these can look quite clean but still require changing – also check exhaust baffle is running clear – see you have an aftermarket on – this can screw up the mixture and make the bike run rich or lean.
I would also get the carb balance checked out with vacuum gauges – if they are out of sync it will still pull strong but wont be efficient
hope that helps