depends what line it is. For sure it might be your carb overflowing, and if that's the case, if tapping it doesn't fix it, it's a fairly easy fix.
What happens when your carb overflows is that your float is failing to push in the needle and seat to close the fuel valve leading into your carb... maybe it's crudded up, or stuck, but sometimes tapping it will loosen the crud or the stick.
If not, take off your seat, tank, and airbox, and you'll see the carbs. drain them and pull them off, and take off the float bowls (four screws on the bottom). This is also a good time to clean the bowls and jets... well worth it, since your bowls are already off.
Anyways, your floats are the big plastic things in there. As your bowls fill with fuel, they float and as they go up they push a small needle up into the fuel valve. as it goes further and further up it restricts the flow of gas more and more due to it's tapered shape, eventually cutting it off completely. Then, as your bike uses fuel from the bowls, the level drops, lower the floats, lowering the needle, and gas starts pouring in again starting the cycle over.
Sounds complex in writing, but a quick look will make it all make sense. It's very self-explanitory.
Once you find the 'needle'--the thing that the float pushes up into a passage--check to see if it's stuck. If it's stuck, you can loosen it by hand and hope it doesn't stick again. I'd say about 50% of the time it does (in my experience). Might even work fine for a month before sticking again, but if it happens again then a tap should fix it for sure. If you don't want to deal with tapping it, get a carb rebuild kit with a new needle.
If it's not stuck, might still be your overflow. Could still be two things that I can think of: maybe your needle is worn, in which case it needs replacing (check this by seeing if pushing it mostly in the passage stops fuel flow). It also might be your float height, which means that your floats are able to go up so high before restricting fuel that the carbs are allowed to overflow. This is a best-case scenario because the fix is easy and permanent: adjust the float height. If i remember correctly, these bikes just have a bendable tang to fix height.
The tang is the little metal tab attached to the float that actually does the pushing of the needle up. You will see it, and you will also see how bending it will affect how high the fuel level is allowed to get before it stops. However, don't just adjust it willy-nilly. There are certain specs to adjust to, found in the manual.
Any questions, just post back. Keep in mind that this info is only for if your carbs are overflowing... these won't help if some other tube is leaking! might help t post a photo or really detailed description of what tube is leaking...