gas tank and carb leak

When I ride my bike,like take a short fast ride which is good enough to warm up the bike, and I do some serious rpms, then park, shut it off and then sometimes I hear the gas tank cover/lid hissing and when i went to check it out this is what happens. I take off the cover and i put my hand over it and I thought it would be sucking in but its blowing out the fumes. Im not sure if it hinders my fuel economy, But is that normal or is the cover supposed to have a tight seal. Plus at night or when I park the bikeI have to set the petcock to On which is only vacuum operated so that my gas doen't overflow and leak out from the engine to carb intakes. does anyone no why its doing that?

del

I set the petcock on on when I am riding, but I have to set it to prime 90 percent before I start it. Also, so the air blowing from out of the tank is normal? I thought it would of been more of a suction.

del

Sounds possible. I put my hand over it to cover it and iI raise my hand off of it and it shoots out. Plus do you think that dirt in the carb would cause it to just leak out gas? I have a new filter which I put on awhile back, Because the bike has been running without one for awhile( previuos owner never got one!)

del

So if i have fuel leaking at the carb holders, that would also mean a possible air leak, rite? I have this excessive idle that goes up to 4000 sometimes. I just finished putting zip ties on the airbox manifolds and I tested it but it does the same thing. How would I seal that or prevent it from leaking, besides buying new holders?

I fixed the problem with the leaking gas through the manifolds really good. It doesn't leak anymore since I dropped it. lolI had a spill and once again I say thanks to my engine guards.