If you're consistently smelling rich fumes while moving I'd have to say it's the venting or lack there of at the gas cap or the gas cap seal. Anything lower or further back would get wafted behind you, not in your face. Your tank gets pressurized and needs the venting.
Agree with the above that we need more info. As it stands right now the answer could be as simple as “quit farting”. So I’ll throw something out here that might help you help us help you. We’re probably talking about tuning. Which is why we need to know what modifications, what tuning device, etc are on the bike. My guess:
The previous owner modified the exhaust or just gutted the mufflers ‘cause he wanted to be a badass and thinks a really loud motorcycle makes him one. On Vic’s this leads to exhaust popping on decel, often mistaken for backfiring. So he got on a forum and found out that putting a tuner on the bike and richening up the mixture (adding fuel) at 0% throttle eliminates the popping so he added a bunch, maybe too much, and now you’re smelling gas blowing out the exhaust.
Here’s the mechanics involved. Your fuel is delivered electronically by fuel injectors. Your air is delivered mechanically via cable operated butterfly valves in the throttle bodies. So when you completely close the throttle the fuel shuts off faster than the air. In the scenario of an opened up exhaust on an otherwise stock bike this results in a wave of oxygen flowing through the engine and out the exhaust where gas fumes + hot pipes + O2 = fumes ignited = a loud POP. Annoying but harmless. This sounds counter intuitive, but if you add a little fuel you get more of the O2 used in the combustion chamber (the engine is still running) which means less O2 in the mufflers for that pop. But add too much fuel and it just gets blown out the exhaust.
So I’m thinking you’re completely closing your throttle on decel, and your air/fuel ratio is too rich there via a tuner / fuel controller, and that’s creating the fumes you smell. Here’s a test that might help identify this condition: next ride when you’re decelerating try pulling the clutch in but do not completely close the throttle. Gently roll off the throttle but don’t completely close it. Is the smell gone now? If so I think I’m onto something. If not then I swung and missed.