Car Talk: Carburetor Leak Could Be The Cause Of Gasoline Going Out

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Dear car speech:

I have a 1957 Chevrolet Nomad with a small block V8, high performance intake and carburetor etc. If I don’t start it for a few weeks, it looks like the gas tank is sucking up all the fuel. I tried installing a check valve, but it didn’t help.

I owned it for 48 years. Aside from installing an electric fuel pump, do you have any idea what may be causing it? Thanks and be careful. – John

That’s the built-in anti-theft device, John. Actually, your theory makes sense, John. But also false. Like many of my theories.

Your problem is not that the gasoline is leaking from the carburetor and back down the fuel line to the gas tank. The problem is that the gasoline escapes from the bowl of the float and evaporates.

Here is how it all works. When you turn off the engine, your carburetor stores a bunch of fuel in the float chamber. This is the fuel that is used the 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 would have to write to me on how to replace a fuel line, so don’t do that.

Either way, it takes the fuel pump several seconds – while cranking and cranking the starter – to get enough fresh fuel into the carburetor to keep the engine running.

Normally that wouldn’t be a problem because like I said you have a lot of fuel in the carburetor until the fuel pump catches up.

But in your case, during those few weeks that the car is sitting, fuel escapes from your carburetor, maybe into the intake manifold, and evaporates. So when you start the car, the float chamber is dry.

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

But you would be smarter to just replace the carburetor. There may be a crack or some other issue that is causing gasoline to leak. And if you eliminate that leak, who knows, you can go from 9.4 miles per gallon to 9.5.

A question about cars? Write to Car Talk Write to Ray in Care of King Features, 628 Virginia Drive, Orlando, FL 32803, or send an email by visiting the Car Talk website at www.cartalk.com.

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