New fuel filter?

On my 79 SN recently it has become harder to start. It seems as though if it sits for awhile, the gas doesn't stay in the line and carb almost as if it's running back down by gravity. My 1st guess would be to replace the fuel filter? The last time out after I got it to fire at the dock, it ran great. But after about an hour and a half at a sand bar it was hard starting again. I pulled off the air filter and it seems like it's dry in the carb. Once running it runs great. Anyone have any ideas as to what could cause this? The previous owner stated he had the carb rebuilt last year? I'm thinking it may be time for a new carb or choke? Thanks, Brian