I'm tempted not to respond, but I'm going to comment on this one last time...
It is possible for fuel to leak out of a carburetor through a defective plug. Said plugs are installed as part of the manufacturing process in cases where a passageway has to be drilled after the carburetor bowl is cast. Drill bits and drill presses being what they are, such holes have to be made and then plugged.
But - if that's the problem, you should see a discoloration around said plug from fuel weeping past the plug. In severe cases, you'll see fuel dripping from said plug. If you don't see said discoloration, you don't have a weeping plug.
It is not - repeat, NOT - possible for a defective fuel pump to siphon the fuel out of the carburetor bowl. The needle and seat are in the air horn, and therefore sit above the level of fuel in the bowl. If the fuel pump has a leaking diaphragm or valve, it will drain the line from the pump to the carb. Because the needle and seat are above the fuel, all that gets sucked out of the fuel bowl is air.
I am 100% certain about the above paragraph. I have an example of every factory carburetor used on tri-fives (BC, 2GC, 4GC, WCFB), and have taken them apart. I've also read the specs on setting float levels. See
RCB Index for the factory data sheets for your particular carb. Read the spec, take the air horn off your carb, and get out a ruler - you'll see siphoning is impossible.
If you argue, "but I'm running a Holley, Edelbrock, 3 Stromberg 97s, a Whiz-Bang Super Duper carb" or whatever - no, I haven't torn apart one of your carbs. But, if your carb has the needle and seat in the air horn, I stand by the above statements. Unless your carb's fuel inlet is near the bottom of the bowl, the leaking fuel pump theory simply isn't valid. I've never seen such a carb design, so if you have one, please post a picture.
For those of you running Edelbrock Performer carbs - take a look at page 20 of
https://www.edelbrock.com/pub/media/wysiwyg/documents/carb-owners-manual.pdf . The needle and seat are in the air horn - above the bowl, and above the level of the fuel.
Some Holley and Carter carburetors have screw-in plugs in the side of the bowl for checking fuel level. The idea is - if you remove the plug and fuel just barely drips out, your fuel level (and therefore your float setting) is correct. Guess what - the bottom of that threaded hole is comfortably below the needle and seat mating surface.
Once again - unless your needle and seat sit below the normal fuel level in the bowl, siphoning is impossible. And, even if they did, the fuel could only drain down to the bottom of the seat. Every carburetor I've ever worked on has the seat above the bowl - not in the bottom of the bowl.
That leaves evaporation of fuel out of the carb bowl. So, let's talk about that.
Every carburetor has a bowl vent inside the carburetor throat at the top of the air horn (inside the air cleaner). Most also have an "idle vent" that sits outside the air cleaner (See my photos at
geekin' out on carburetors ). The air pressure differential between the vent(s) and the venturi is what forces the fuel out of the bowl. Without said vent, the carburetor wouldn't work. Unfortunately, that vent also gives fuel vapors a path to escape.
I have to crank my '57 (283 with WCFB) twice before it'll start if it's been sitting for a few weeks. If it's been sitting overnight - one pump of the accelerator, turn the key, and bang, it starts. When sitting overnight, it starts faster than my modern cars with fuel injection. When sitting for a few weeks, it starts faster with the WCFB than when I used a 4GC, because the fuel bowls on a 4GC are much bigger than a WCFB.
The other consideration is fuel itself. We think of fuel volatility as a bad thing. It's not the case. In fact, it's the more volatile components of the mixture we call gasoline that actually allow us to start a cold engine.
Last week, I attended a session on carburetors given by a retired Rochester carburetor engineer in our car club. He told of participating in some extended testing of cold-start scenarios, where all of the "big 3" brought their cars and engineers to a testing site with all the major oil companies. They tested cold starts with low, medium, and high volatility fuels at precisely 40 degrees ambient temperature.
The test was - when the outside temperature reached 40 degrees in the morning, start the car, idle for 5 seconds, put it in drive, and floor it. The result - high volatility fuels hands-down won in that situation.
So, even if the fuel in your carb bowl hasn't completely evaporated - if only the higher-volatility components of the fuel have evaporated, it'll be harder to start.
Whew - that was longer than I'd planned...
