Clothes Pin On The Fuel Line?

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It doesn't/can't but people keep saying it.
It can only drain back from the needle/seat but it's impossible for the gas in the carb to be sucked out.
Heat evaporation is what usually empties the bowls.

An electric pump and fuel return line to the tank solves all the vapor lock and drain back issues.
In RRR's case the check valve just keeps the fuel from running back when the engine isn't running.

I am in complete agreement.

Not a slant six, but an 88 Diplomat with a 318. I have had a continuing problem with hard starting and/or vapor lock, after replacing a mechanical fuel pump that broke the arm, at the pivot. Found I only had 2.5 lbs fuel pressure at idle, and took too long to pump from the tank. Had this problem with 3 pumps from different sources. Checked the FP eccentric with a camera, and it was fine. Turns out all the fuel pumps had excessive free play in the stroke, and not enough "pumping" stroke. Cured the problem by slightly bending the pump arm, to reduce the freeplay. Now have about 4.5 -5 lbs fuel pressure, and no more vapor lock, or hard starting
 
Of the many different molecules that gas is comprised of, they all have different rates of evaporation at a given temperature.The most volatile are called VOCs; Volatile Organic Compounds. It these that start the fire in your chambers. In an EFI closed system, the VOCs can't get away, so your gas stays fairly fresh all week.
But the same gas in a carb, is a different story. The VOCs in the float bowl are gone; some in hours, some overnight, and the thicker stuff that is left, is hard to light even in an 8/1 low-compression engine. So the engine cranks and cranks until some fresher fuel comes into the bowls with enough VOCs to initiate combustion,..... all the while you are thinking the gas is gone. And some of it is, but what's left just won't combust at 8/1.
To prove this, just store a couple of ounces in an open-top container,under the hood overnight.Take note of the color. About 24 hours later,note the color,and pour it out on the garage floor; then try to light it with a spark. Yeah good luck with that. It will likely take an open flame to start it, and it will smoke and stutter and leave a greasy stain on the floor. I used to say, by the end of 7 days, at room temperature, you can't even light it with a flame.
People call modern gas crap, but think about it, your EFI has no trouble lighting it even after two weeks, thanks to it's closed system, trapping the VOCs. Well that and the finer atomization of the injectors,lol.
IMO, there is nothing really wrong with oxygenated gas that cannot be overcome. I have been burning it in my carburated 367, since 1999, when it first came to Manitoba, with no special fudging.
But I do notice in my 80Volare which sometimes sits for weeks, she's not happy about it. For her, I keep a small bottle of stabilized gas under the hood, and just dribble a squirt down the top before hitting the key, and Vroooom away we go. The bottle is clear plastic, and I don't change it until the gas turns color, which, in the sealed container takes months and months.

You can tell how stale the gas is by it's color. In my experiments;
Clear is fresh
Yellow is aging but will still run well, even tho starting is becoming harder
Orange shows the Vocs have mostly evaporated. It will run ok around town, but will definitely be hard to start, and power is lazy. You may experience hesitations and such.
Pink is losing power fast and will likely need aux fuel to start your car, and driveability/power is way down.
Red may not even lite with an open flame, and if you do get it lit will smoke and stutter and go out before being consumed.The Vocs are long ago gone.
If you leave it long enough it will become very dark red almost black, and a thick sticky syrup.Fresh gas will not dissolve it. And it's hard to get off your fingers if you dip into it.It will be difficult and expensive to remove from your system.
 
Back when I did run common cornohol fuel I had all sorts of fuel delivery problems. When I put the 73 dual plane 2brl intake and carb on my 67 273, I placed the fuel filter vertically by the timing cover just as it was on the 73 318. That along with the thickest available carb base gasket cured my percolation problem. Today there is plenty of ethanol free "real gas" available locally so I run that.
I had this problem on a 1967 Jeepster Commando W / 231 Buick engine. The thick gasket under the carb fixed my problem.
 
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