I have an 81 Lebaron with a 225. In the past, sometimes there would be a slight hissing sound when removing the gas cap, but lately it's been more consistent and increasingly louder. I'm almost nervous to remove it now. Yesterday, I went through a lot of gas on a short drive, and a few times it would stumble getting up to speed. Once I was cruising it was alright. I figured it got about 10 mpg, which is way below the normal ~17 that I usually get. I was having the stumbling issue about two months ago, and I replaced the fuel lines and filter under the hood, which took care of it. I also rebuilt the carb at the time, a Holley 1645. I haven't had the car off the ground yet to look at the lines at tank because of the Michigan weather. I've been keeping an eye on the ground beneath the car, and I haven't seen any leaks yet. What do you guys suggest?

thanks!

new gas cap

My AP6 was doing the same thing.Turned out to be the pressure release line from the filler neck,a mud dobber (wasp) had decided to build a nest in the end of it.I took it off where it connects to the filler neck and blew it out with my air gun,no more pressure build up.On the AP6 the the pressure release line goes from the filler neck,through the boot/trunk floor and sits beside the petrol tank underneath,it doesn't connect to anything .
I don't know if this is how it is on your Lebaron.

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DRIVE IT,DON'T HIDE IT

You may need a new gas cap, but probably not; the primary fuel tank vent is via the charcol can in the right front corner of the engine bay. The overpressure relief valve in the fuel cap is only there as an emergency backup vent. You'll need to follow the tank vent line from the back of the car forward to the charcoal can, inspecting all hoses and pipes for incorrect routing, blockage or damage and repairing as necessary. The can itself may be in need of replacement.

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thanks for all the info. the charcoal canister isn't even in the car right now. i removed it a few weeks ago, as all the vacuum lines going to it weren't even hooked up. the fuel line that was supposed to go to it is still attached to the pod it was on, but the pod isn't attached to anything. i kept all the vacuum lines and the canister, though, so if need be i can put it back in.

in a move of sheer ignorance, i failed to realize that the pod the vent line is going into is vacuum powered, so it's probably been closed this whole time. i took it off; we'll see if any pressure builds up after I drive for a little bit.