There could be several reasons for this – but I suspect the most likely cause is the vent. The vent is actually the charcoal canister and is located under the hood passenger side front corner.
There will be either 2 or 3 ports on the Charcoal canister. The small port (small vacuum hose) goes to the base of carburetor and it helps to remove the fuel vapor from the canister (ie: pulls the vapor into engine to get burnt).
OF the two large ports, one goes to the fuel tank and other goes to the top of the carburetor. My ’77 has no large hose going to carburetor and that port is plugged on charcoal canister (from the factory) – but most FMJ’s will use that carburetor vent (it catches any fuel vapor from carburetor after shutoff and stores it).
What I recommend to do is to locate and disconnect the large hose that goes towards rear of car from the charcoal canister – and then fill the fuel tank. If tank fills better, then the filter on bottom of charcoal canister is dirty (or canister is stopped up). In many cases, the filter can be removed from charcoal canister and cleaned or replaced – but that is not the case on all charcoal canisters.
Note: the owner’s manual on my ’77 Wagon says the charcoal canister filter (called a vapor storage canister filter element) is to be replaced every 30k miles (48 km) – but I don’t recall ever changing one as part of normal maintaince.
If disconnecting the hose did not do the trick – then a person will need to get car jacked up some distance of the ground (you will need plenty of wiggle room under the car). Start to trace the three metal fuel lines going from front to rear of car (5/16” will main fuel supply line, 1/4” will be fuel return line and 3/16” will be fuel vapor line (aka the fuel vent) - which is the smallest of the 3 lines). Check for bends, kinks and for rust. I prefer to use a small flat screwdriver, but an ice pick also works. Anyplace you see that might be rust – poke it. If your “poker” dents or goes through the tube – then you have a rust repair to make (and maybe more). These tubes can rust from inside out (and outside in) and rust will restrict airflow through an already small line.
The rubber hoses can also come apart and a small flap inside of hose can restrict air flow (or fuel flow). Sometimes the rubber hoses that do that are generally also cracked on outside. If a car has sat outside for 25+ years – it is not uncommon for those hoses to be on their last leg . . . several years earlier.
A few years ago, my '86 Fifth Ave was getting poor mileage and hard to start in morning (excessive crank time). Come to find out the “hidden” hose (by where the firewall is at on passenger side of car by frame rail, a commonly overlooked hose), the main fuel hose had cracked to point it was leaking fuel after car was turned off. I changed all those hoses and several problems was fixed.
Note: 3/16” fuel hose is expensive!
I needed just under 3 foot of each size fuel hose – but your vehicle might vary. Measure first (at tank, the hidden area and underhood (to fuel pump, to fuel filter and to charcoal canister)!
It might be helpful to have a helper when working on the main fuel line hose. That line is full of fuel and you “could” siphon the entire tank if not careful. The fuel return line is also full of fuel, but it will not matter if any leaks out (except for the minor loss of fuel and any that gets in your face).
A rusty line repair can be done on car – but you will need to pull line out of the way some, get a pipe cutter and cut a decent section of line out and replace with a section of rubber fuel hose. This size pipe cutter works great on-car.
Hopefully this helps.
Currently, I have no problems filling the fuel tanks on either car (’77 wagon or ’86 Fifth Ave) with the exception the fuel filler nozzle wants to fall out of the wagon when in use. I have to keep my hand on it when filling – which is not a concern. The concern I have is the fuel filler hole is down lower than I like, and it requires for me to bend down when filling – which is, um, uncomfortable to me.
BudW