I’ve had that problem a few times over the years too. The symptom however is a visible fuel leak from the accel pump housing. No such leak in this case.
The 2021 case was solved by a change to the fuel bowl venting. That change remains in place. Or do you think the fuel bowl isn’t evaporating, but leaking instead?
So now the vent is open to the air, fuel will evaporate out that vent
To weigh in here from the possibly related perspective of SU carbs, empty fuel bowls after sitting can be the result of evaporation but more likely, especially with an electrical fuel pump, the result of a sticking fuel float or a sticking fuel intake needle.
As the fuel evaporates after sitting, after evaporation the float sticks in the “full” position, now not allowing fuel to flow into the now empty bowl.
Alternatively, , the fuel lever and/or the fuel intake needle valve is "gummed up " preventing fuel from entering the carb.
The solutions are…
- Periodic cleaning of the fuel delivery with something like Chevron.
- Periodic cleaning of the carb.
- Serious cleaning orrestoration or replacement of the carb.
.
It could be just a worn out needle valve letting it leak out.
I don’t know anything about carburetors, but I’m happy to hear that your early 90’s Corolla is back on the road, and hopefully running well. I saw a white 1990 Corolla on the road earlier this week, and it made me smile to see that classic car in my daily travels. And I know it was a 1990 model, because it had the seat belt buckles on the front doors, but the front grille just said “TOYOTA” instead of the “T in a circle” logo which debuted for 1991.
My first car was a 1988 Toyota Corolla, and it really was an excellent car, even by today’s standards. It was very comfortable, fuel-efficient, easy to drive, and easy to park. Mine had the 1-barrel carburetor, which required annual servicing and adjustment to run properly. An optional fuel-injected engine was available, but unfortunately, not requested by the original owner.
I would not mind having another one, but it would need to be the 1989-1992 version, which is fuel injected. I’d never own a vehicle with a carburetor ever again, too much hassle.
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When was the last time you rebuilt the carb? This could be part of the preventative maintenance plan you write about and might help solve this problem.
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+1
If you are going to take a $40.00 perfectly working axle out of a car and rebuild it for preventative maintenance, why have you not taken the carb apart for inspection cleaning and rebuild, if the gas is disappearing???
If you think that amount of fuel can evaporate through the tiny hole of the vent nipple, what makes you think there would be liquid fuel in the catch basin if it was slowly leaking through the gaskets or a porous casting and evaporating as it leaked out?
FWIW, I have doubts about the evaporation theory. Many carbs back in the day sat for a week without the fuel evaporating completely from the bowl. I have carbs on older cars that have sat for months and gas will still spill from the bowl when they are removed.
Often, gas will leave streaks across the surface it is leaking past. Look carefully at all sealed surfaces and maybe you can see if there’s a leak somewhere?
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@Mustangman gets the thread solution, +1. @weekend-warrior offered-up some inightful ideas as well, to which I grant runner-up status … thanks to all for the ideas.
Is it fixed or do you just like what you read?
Just be very sure you install the electric fuel pump correctly, with all safety measures. You don’t want it to keep running when it shouldn’t. Fixing/replacing the carb would be easier.
I seem to recall a power valve i n the bowl that can fail and leak down the bowl.
Yes, I’ve had that happen. That leak goes from the fuel bowl into the intake manifold. But it shows up in a carb leak test where— as an experiment — you fill the fuel bowl then place the carb over a container overnight to watch for any leaks.
When I bought my very old, very used 1979 Toyota 4X4 truck, it had a small electric fuel pump mounted on the tank. It ran whenever the key was in the On position. A plate covered the place where the OEM engine-mounted mechanical fuel pump had been located.
I never figured out how to have that electric pump run only when needed, so it did present an increased fire hazard in case of a crash. The truck’s demise was quiet: the frame broke behind the cab when it was being lifted to rotate the tires. RIP.
My 1983 Corolla sometimes needed a tablespoon or so of gas poured into the carb after it had sat for weeks or months.
Whenever I see the addition of an electric pump to a mechanical one it reminds me of the trouble my brother had with his Jeep. He was having stalling issues especially at altitude and steep grades. So a 4x4 shop installed a booster electric pump which seemed to work. However, it had an unpleasant side effect where it would seriously flood the engine if the carb angle was such that the float didn’t close and fuel spilled through to the engine. The electric pump would happily pump away and almost hydro lock the engine. After being stranded several times, he bought a TBI kit I installed for him and never had any issues after that. George’s truck is unlikely to see that kind of scenario but just made me think of it…
The truck is 50yo, why does it all of a sudden, after 50 years of service with a mechanical fuel pump, need an additional electric fuel pump now??? Wouldn’t just repairing the carb be cheaper?? 
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Fair question. Let’s make it a puzzler: What’s different now w/this truck than a few months ago? Hint: answer already posted above.
5-7 days is too soon for the fuel bowl to dry up from normal evaporation. Something is wrong with the carb.
What is the normal time between uses the fuel level should remain high enough so the engine starts with no more than 5 seconds cranking? In a 50 year old design?