After you have made sure the fuel line itself is not obstructed, by blowing through it it while it is disconnected from the tank, reconnect it to the tank and blow through it again as you have someone listen with his/her ear to the fuel filler or the top of the tank inside the trunk. If he/she doesn't hear a bubbling sound, your pick-up tube may be plugged tight and may need higher pressure to clear. If that does the trick, you will still have to have the tank cleaned to keep the problem from coming back. The other possibility is that the pickup tube is corroded through above the fuel level. Filling the tank to the top may get you above the corroded spot and bring temporary relief, but you will have to replace the pickup tube. You can do the cleaning and/or tube repair yourself, BUT YOU HAVE TO BE VERY CAREFUL HANDLING THE FUEL!!!!. DISCONNECT THE BATTERY. Drain the tank through the drain plug, out in the open air away from sparks and running electrical equipment. Remove tank and clean it with hot water and detergent. Allow to dry. You can help the drying process by blowing air though the tank with a compressor. If you don't have a compressor, place the tank with its top opening down on a couple of saw horses and let it stand like that overnight. Fuel vapor drains down. When you are absolutely sure there are no fumes left in the tank, you can scrape around inside and pick up the scrapings with a vacuum cleaner. If you need to replace the pick-up tube, unsolder the connector with a propane torch(FAR AWAY FROM THE STORED FUEL) or a hot soldering iron (safer), cut off the corroded part of the pick-up tube, leaving a stub, and solder a piece of bendable copper refrigeration tubing onto the remaining stub.
I hope all this will not be necessary, but that's how I dealt with a similar problem.
Bob S.