Areas I would check (if not already done so):
- Rusty or damaged metal fuel lines (all 3 tubing sizes / 8 sections of tubing).
- Cracked rubber fuel hose (3 different sizes (5/16”, 1/4” and 3/16”) located at fuel tank, at close to passenger side firewall (kinda hidden location), at fuel pump, at fuel filter and at charcoal canister. A leak at any of these locations can cause problems – and sometimes a cracked hose will not leak fuel but will suck vapor (easier to suck air than fuel) instead.
- Make sure the air filter on bottom of Charcoal canister is not stopped up. Take a section of hose (all 3 sizes, once for each port) and blow (don’t suck, unless you like to inhale fuel vapors). Note: you will need to plug 2 of ports when blowing through the 3rd port. Some, to a lot of, restriction will be present. If you can’t blow – then you have a problem with it.
- The fuel tank is vented through the charcoal canister. Before messing with gas cap, first take a drive in which you can tell it is acting up. Remove the gas cap (or let it dangle if it is tethered to car) and take another drive under same conditions as above. If there is a noticeable difference – then you need to recheck the above items.
Note: I don't recommend driving a lot with gas cap off - for dirt is enemy #1 for fuel systems. It is OK for testing purposes.
Note: a car with a controlled vent system will get better fuel mileage than a car that has an uncontrolled vent system. Not a lot better, but anything better is always good.
My ’86 Fifth Ave, after a long trip, will percolate the fuel from the carburetor from hot soak after turnoff. In those cases, the fuel vapors travel down to the charcoal canister (on cars without a bowl vent, like my ’77) or will stay in the bowl (on cars with a bowl vent solenoid). My ’86 Fifth Ave currently has a ’77 318 Carter BBD installed on it, instead of a computer controlled Holley version (so no bowl vent). On cars with a (working) bowl vent, the evaporated fuel vapor should condense back to liquid fuel once the carburetor cools down.
Same thing happens (sorta) with fuel tank. Gasoline will evaporate at room temperature. On cars with an open vent, those fuel vapors leave the car never to be seen of again. This is not a fast process – but it does happen, more so on hot days.
Cars with a regulated fuel vent (most FMJ cars), the fuel vent goes to the charcoal canister. The charcoal soaks up the fuel vapors - instead of it going to never never land. When engine is running, there is a small vacuum hose to carburetor that applies a small vacuum on the charcoal canister which allows those trapped fuel vapors to get burnt.
I’m not really fond of the complexity nor the “clutter” of the EVAPORATIVE system, but it does work, and it is required in some states or city’s.
That said, if performing a major overhaul of engine or performance upgrades (for street usage), I will leave the evaporative system in place because it does help a lot more than it hinders.
Don’t believe that fuel evaporates at room temp?
Take a dish outside (not something from kitchen, or your better half will remove some of your hide, or plastic) and pour a little gasoline in it. Come back tomorrow and you may find it’s all gone due to evaporation. Same thing happens to water, to a lesser amount.
BudW