[dsraven]

another thing that is common is that the hot engine will boil the fuel in the carb after you shut it off and walk away. it usually makes the thing smell like gas (and also the garage), but if you park outside and don't hang around the vehicle you may not notice. a thick fibre or phenolic spacer gasket under the carb usually helps insulate the carb aginst engine heat soaking it.
try the "look down the throat while pumping the gas" trick first. it's cheap and easy. try the clamp on the fuel line trick second. it's also pretty cheap and easy. for the heat soaking thing you gotta be hanging around after a "hot shut down" to smell under the hood for that raw gasoline smell. a fuel filter with the check valve is a great idea and may solve a siphoning issue. it's pretty easy to see if it is a siphoning or run back issue though