Vintage Cars: fuel smell and cosumption, vent system, gas tanks

Question
hi my car is a 280se straight 6 and ihave a problem of every time i start the car the smell of the petrol is inside the car and gets worse when i put the heater or fan on.If checked the fuel line and there is no leakage

Answer
heater or fan draws air from the front cowl and from the engine bay.  It does not take a lot of "leak" to create quite a smell
Is there any wetness under or around the carb.  Is the smell there before you start it?  May have a flooding issue, or overflow into carb bowl, does it start easily?
Also the gas tanks have a vent system try leaving the gas tank cap a little loose and see if the smell gets better.

Good Luck
Ron