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Old 03-16-2012, 02:23 PM
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gas leaking/evaporating?

I have put on 3 different carbs. and 2 different fuel pumps on my 56 283 engine, and the 56 chev. does not have any gas venting, such as vented gas cap. The gas must leak or evaporate or be sucked out of carb. somehow. I have searched for months for any signs of leakage from carb/lines/ tank/pump and find nothing. Car sets for 2 days without starting, and I have to pump and grind over and over for it to start. I have taken all 3 different carbs off car, then filled up bowls and let each one set in cool garage, and after 2 or 3 days the gas is still in bowls. The car starts pretty good if I dont let it set more than 8 hours. I know this seems strange, but it almost seems as though
the gas is being sucked back through fuel pump. The 3 carbs are a 2bbl. holley, an edelbrock 600 and 2 quadrajets, which I peened and JB welded lead plugs on both of them. This thing has really got me stumped. Can a fuel pump create a vacuum and accually suck gas back from carb? Thanks for any help

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Old 03-16-2012, 03:48 PM
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Do you havea working choke connected? The fuel pump can't suck the fuel up within the bowl, through the float valve, and then back to the pump , meanwhile pushing more to the tank.
And, I've gotta believe your tank, or your cap, is vented. One way or another. Even if it's just a bad gasket. Without venting, pulling fuel from the tank creates a pressure differential, and the tank will collapse.
I think setting up a good working choke should be a big help.

Pat
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Old 03-16-2012, 07:45 PM
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All 4 of these carbs. have good working chokes. Thanks anyway for your reply.
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Old 03-16-2012, 08:32 PM
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Are you saying that the float chamber emtys out after 8 hours ?
Try this, an old flathead trick. Gun the engine and kill the ignition while it is still reving. Supposed to suck some gas into the cylinder and stay there until the next startup. I have done this with my 350 and come back 3 months later and couldn't believe how quick it fired up.
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Old 03-16-2012, 08:51 PM
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I Agree With The Fuel Pump Not Being Able To Suck Gas Through Bowl And Float Needle, I Am Just So Frustrated That I`m Making Statements
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Old 03-16-2012, 08:57 PM
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DBFlyer
You've tried 4 carbs. You say they all have working chokes. Perhaps the engine is trying to start, consuming the fuel in the bowl, and the pump has a leaking diaphragm allowing line fuel to leak back to the tank.
When the engine runs, does it starve for fuel under WOT?

Still trying

Pat
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Old 03-16-2012, 09:21 PM
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I live in Atlanta, take my hotrod out just about every weekend. I spin the motor over to build oil pressure, pumping the gas to wet the cylinders, then add the ignition and it fires.

A sitting car, fuel can evaporate from the carb. You live in AZ. hot and dry, the cylinders dry out also, it can very well seem hard to start. People do not realize just how often they drive their cars, and saying that it keeps the motor, for lack of a better word ready.

You sure on the 8 hours or just so aggravated it seems that way. If that is so then that sounds like a lean condition and one pump with the peddle will cure that. Again dry fuel system, and by fuel system I am including, the engine internals, manifold, cylinders, no fuel no fire. If you use a choke recheck the adjustment, you do realize that even with a choke, electric or by metal you need to depress the gas peddle so the choke will set, which by the way sends some fuel into the manifold. Without a vent in the tank, it won't collapse the tank but it could create a vacuum that would inhibit the fuel flow.

It is normal to experience what you describe, first it is a hobby car and sits more then a DD. The FI cars over come this because of the design.

Just my .02, but most carb, with mechanical fuel pumps will act as you describe if allowed to sit.
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Old 03-16-2012, 09:46 PM
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I think I would drive it,let it sit for a couple three days and carefully pull the top off the carb and see if the bowl is empty or full without trying to start the engine. If you still have fuel you might look at your ignition system.

Are you running a aluminum intake by any chance? I had a BBC years ago with a medium high rise Corvette manifold that would generate enough heat to percolate the gas after shut down. Fixed it with a phonolic spacer and a flat black aluminum heat shield between carb and manifold. Are you cooking the fuel out of the bowl after shut down? Any fuel vapor curling out of the carb? To check that wait a couple of minutes after shut down, take the air cleaner off, hold open the throttle wide open and wait a few minutes to see if the vapor cloud starts creeping out.
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