Take the tank off and check the tap. It sounds like you haven't got a manual - so get one. In the UK a Haynes will set you back £22 or so from a shop, a bit less from an online store, and if you search Google enough, you might come across a handy website that has just about every one ever published as a PDF download. Of course, that would be classed as stealing so I;m not going to post a link..
To get the tank off, you need to remove the rider's seat, which if the original spec bolts are fitted is held on by a pair of dome headed bolts with a 4mm allen socket. You also need to unbolt both seat side panels where they are held on at the lowest point of the panel. The bolts, if original, have a much bigger head, but are the same size allen as the seat screws. With those out, you can ease the lugs at the front of the panels out of the grommets on the fuel tank. It is not necessary in my experience to remove the seat panels, but you can do so if you wish. To do this, you'll need to release the pillion pad using the key, then unscrew the safety wire from it's base so you can fully remove it. After this, gently lift the tail to release the lugs from the grommets at the back of the bike. Do not lift it too far, because the rear lights need to be disconnected before you can take the tail away. The wiring runs up the left side of the subframe, and there will be a large rubber boot somewhere close to the end of the tail. Inside this, you will find a flat connector which needs to be disconnected. There is a clip on one of the thin edges to release it. With this disconnected, you can take the panels away.
The tank is held on by a single bolt at it's rear. I think it's 12mm, but I never really look at the numbers on spanners, I choose them by eye. Take the bolt out and lift the rear of the tank. This bit is a pain in the arse, but you need to stand at the right side of the bike (opposite to where the fuel tap handle is), lift the tank a little with one hand, and use your other hand to disconnect the fuel pipe from the tap. It has a butterfly clip on as standard, and to be frank, it's a right bastard to shift this, and you're guaranteed to end up with nice red lines and probably cuts on your hand while doing so (or at least, I manage it... lol). Once the clip is slid down the pipe, you can ease it off the spigot. A little fuel will be spilled even if the tap is working correctly. You then need to remove the small, thin vacuum pipe from the tap. Once this is off, the tank can be removed. You need to rotate it a little to avoid catching the fuel tap handle on the left side of the tail if you haven't taken the seat panels off.
If the tap is working correctly, and hasn't been modified, fuel will NOT flow at all from it when it isn't on the bike, no matter which position you turn the tap. If it has been modified as per the HRC manual (or in the rather unlikely event of the diaphragm sticking open), fuel will flow when ON or RESERVE. OFF will still seal it. If fuel flows on the OFF position, it's knackered and it needs fixing or replacing. To get fuel to flow if the tap hasn't been modified, you need to apply a vacuum to where the the little pipe was connected to the tap. It's a safety measure. This pipe runs back to one of the inlet tracts on the engine which means the tap will only deliver petrol when the engine is running, regardless of the tap handle's position. It is to stop fuel being delivered to the carbs and potentially flooding the engine while it's not running should there be an issue with the carbs.
If your tap is leaking, your petrol smell in the engine could possibly be fuel leaking past a float valve in one or more of the carbs. Worst case scenario is an engine literally full of petrol, so ignore the symptoms at your peril!
