Leaking Carburetor May Be to Blame for Disappearing Gasoline

Dear Car Talk:

I have a 1957 Chevy Nomad with a small block V8, performance intake and carburetor, etc. If I don't start it for a couple weeks, it seems like the gas tank sucks all the fuel back. I tried installing a check valve, but that didn't help.

I've owned this for 48 years. Other than installing an electric fuel pump, do you have any input on what may cause this? Thanks, and stay safe. -- John

That's the built-in anti-theft device, John. Actually, your theory is logical, John. But also wrong. Just like lots of my theories.

Your problem is not that gasoline is flowing out of the carburetor and back down the fuel line to the gas tank. The problem is that gasoline is leaking out of the float bowl and evaporating.

Here's how it all works. When you shut off the engine, your carburetor stores a bunch of fuel in the float chamber. That's the fuel that's used next time you start the car. In fact, if you severed the fuel line that comes from the tank, your car would still run for a good minute or more just on the fuel stored in the carburetor. But then you'd have to write me about how to replace a fuel line, so don't do that.

Anyway, it takes the fuel pump many seconds -- all while you're cranking and cranking the starter -- to get enough new fuel to the carburetor to run the engine. Normally, that would be no problem, because, as I said, you've got plenty of fuel sitting in the carburetor until the fuel pump catches up.

But in your case, during those few weeks that the car sits, fuel is leaking out of your carburetor, perhaps into the intake manifold, and evaporating. So, when you go to start the car, the float chamber is dry.

Now, you could, I suppose, install an electric fuel pump with a switch on the dashboard. And you could turn on the switch a few seconds before you want to start the car. And that would fill up the carburetor.

But you'd be smarter to just replace the carburetor. It may have a crack or some other problem that's causing gasoline to leak out. And if you eliminate that leak, who knows, you may go from 9.4 miles per gallon up to 9.5.

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