petrol coming out front overflow?

Can anybody help fitted a carbon tank to my bike but when i fill it up petrol comes out the front overflow tube onto the fourcourt!
Can't understand this & why do they have these stupid holes. Also if i lift the tank up when full it comes out the back overflow but it done this at the back with the original tank also?
Can any body tell me how to stop this happening?
cheers

mine did that when I fitted the MH airfilter.
When I placed the tank back on the bike, I pinched the rear vent line (rear of the tank right side).

What do you mean you pinched it to stop it overflowing? or you had pinched it which caused the front to overflow?
ps my rear one doesnt have any tube on it? never has just the little black L shaped nipple in the tank!
CHEERS

On my case, once I lowered the tank into position, I bent the rubber hose that comes from the L shape fitting going into the tank. Once the vent got blocked, everytime I left the bike in the sun (think south Florida), It started to piss gas from the front vent and down thru the tube into the ground (right by the battery).

Cheers for the reply but since i dont have any tube on my nipple at the back of the tank not the cure for me!
I did try blow some iar up the back nipple but still it didnt help?
Everytime i move the bike it drips pertrol from this tube from the front nipple??????
Why are they even their cant get my head around how they work or why the one at the bottom doesnt laek all the time?

I haven't had any issues with the front, but if I leave the block on my rear port (for when I have my tank up) It pressurises my tank, which is especially noticable on cold mornings with a low tank. eg. Won't start or stalls on idle when loading the clutch. Took me a few rides to figure it out. I was going nuts! I'm about to fit a larger tank and will make sure I never leave it on as I'm sure the larger surfece area will make it easier to distort the tank.

Do a search here and you'll find that it's not an uncommon problem - the front overflow is connected to the filler cap assembly by an internal tube and is intended to drain fuel overfill/expansion - it can, however, leak fuel for a number of reasons - mine did this briefly, solved by partially taking apart the spring plate assembly on the cap and blowing through the vent route both ways with compressed air - others have solved their problem by removing seal(s) in the fuel cap.

Cheers for the replies found out the problem! .Filled the tank up, Blew air up the tube inside the tank where the rubber overflow hose attaches to the vent hole under the filler cap it had air coming out!!
so drained the tanks and applied some chemical metal over the joint let set. Blew air in tube & put my finger over vent hole under tank no loss of air!
Filled up tank no leak out overflow . Hope this lasts
cheers

I would not have thought that blocking off the vent/drain was a good idea.........................

I thought he was saying he's just sealed a leak in the tube where it joins the top of the tank, not actually blocked it off completely.