I have a '79 240 that runs great until it has less than a half a tank of gas. After that it will it will miss or "chug" during acceleration. Fill it back up and it runs fine. I also always smell gas after driving a couple of miles but I can't find any leaks. Any ideas?
Either the pre-pump has failed, or the hose that connects it to the siphon tube has disintegrated. If you look under the carpet in the trunk there's a cover you can remove, under it there's hoses going into the top of the fuel tank and a cover secured by a bung nut. Get under the car and disconnect the coupling in the fuel line and then remove the bung nut and pull the pump and sender assembly out of the tank, I recommend replacing the rubber coupling with a piece of new fuel hose regardless so you won't have to think about it again. Wash the pump out with water, blow it dry and then connect it to power (6v lantern battery will do) it should start reliably, if it has any trouble, replace it.
Couple of additions: You can actually hear a running pre-pump by putting your ear against the fuel filler tube (cap off, of course). In my climate, the top of the sender rusts and the pipes will be very thin. All the connections will be rusty, and when you put a little force on them, then the pipes break and your car isn't going anywhere until you get a new sender unit.
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