You do not say what year car you have, so there are a few different possibilities.

The carburetor can crack between the float bowl and the venturi, draining the float bowl over a few days or sooner, along with causing some interesting driveability problems. The accelerator pump diaphragm can leak. If the car is a 1979 or 80, the other diaphragms could be leaking. Or, of course, the needle and seat might not be opening all the way because the float level is set incorrectly or the fuel pressure is too low...

It is not normal for simple evaporation to drain the float bowl in three days. I would expect more like a week in my climate. But if you live in a warmer climate than I do, or if the engine compartment is exceedingly warm when you park the car and/or if you have one of the above problems, I can imagine the float bowl evaporating in 3 days.

The float bowl vent is normally open to the charcoal canister except under certain conditions. I do not remember them without examining a carb, but if you hold it in your hand it becomes obvious which throttle positions operate the linkage to have the bowl vent open and closed.

During normal operation the charcoal canister system should not have a strong enough vacuum to suck fuel from the float or fuel tank... Do you have the vacuum hose connected to the correct nipple?

I hope these ideas help you identify your problems.

Chris Obert
C. Obert & Co. (formerly Fiat Plus)
2131 D Delaware Ave
Santa Cruz, CA 95060 USA
(831) 423-0218
(831) 459-8128 Fax
(800) 500-FIAT (3428) Orders Only
FiatPlus@aol.com
webpage: C.Obert & Co.

Subj: Re: fuel evaporation
Date: 7/23/02 5:21:35 PM Pacific Daylight Time
From: williamnelson@earthlink.net (william nelson)
To: GenObert@aol.com

Thank you for those ideas! I live in Arizona so it's very dry, and on my carb the linkage to the float vent valve is busted so it's always open. I have plugged it now in such a way that I hope enough air can get through to maintain pressure but prevent evaporation..we will see. That problem has now moved lower on my priority list because...

I now have a more serious problem, namely my idle circuit seems to not work at all. I have to close the idle mix screw all the way to drive, and I can disconnect the idle stop solenoid without any effect whatsoever. As far as I can tell, everything works, including the solenoid, the jets are clear, passages are clear...I just don't get it.

The idle mix screw was previously bent and I suppose it could have damaged its seat in the carb....but still I am baffled how this could create the symptoms I am seeing.

This was serious damage indeed. In fact, you probably need a new carburetor, as the damaged seat for the idle mix screw has no doubt created a hole that is too big to allow the carburetor to run properly.

Backing up to your first paragraph, you shouldn't expect your car to run right if you leave the float vent valve busted! It needs to be fixed correctly, rather than just plugging it off! As to your idle system, you should be able to screw the Idle Mixture screw in all the way to make the engine die. The idle circuit only comes into play when the car is idling at or under 800 rpm; any higher idle causes the primary circuit to come on. The holes in your carburetor will get stretched and distorted as the car continues to run without the solenoid being tight. And a hole in the jet will enlarge over time as well. All in all, sounds like your carb is past-due for the recycle pile.