I have had my 68 400 convert since 1978. I had the qudrojet overhauled 8 years ago by someone who I thought was knowledgable. The car got NO kind of gas mileage (10 mpg on the hiway at 2800 rpm's) and the performance was terrible! The lead plug in the bowl was epoxied at the time. However, the fuel bowl always emptied after a couple of days and no raw gas was getting into the crank case.

I finally took the car to a long time friend and nationally known 442 Olds drag racer. After another overhaul, the car runs GREAT and the mileage is way up (wrong float level, primary jets way to big and secondary metering rods oversized). The mechanical advance was also sticky. The lead plug was also epoxied again with a high-tech epoxy

The problem is that the fuel bowl still empties after the car sits a few days. Fuel pump preasure is good but I had heard years back that the fuel pump may allow the gas to be pulled out of the carb and back into the fuel tank.

Has anyone ever heard of a bad check valve in the fuel pump that allows this to happen? I hate to throw a pump at it without input.

The gas is evaporating through the bowl vent in the top of the carb. Mine does it as well, have to crank about 10 seconds or so (i know, seems like forever) but that's what happens when not driven daily.

I have the same problem. After two days you have to crank it a while. I tested it with methanol free gas (kind you used to buy years back) and still have the problem. I even set two spare carbs up on the work bench and filled them with gas, checked three days later and still had gas in the bowls.
I suspect it's draining back to the pump. Putting on a new pump today and will see if that cures the problem.

I have a '69 with an Edelbrock aluminum intake and carburetor (basically a Carter AFB by Edelbrock). After it sits for a few days it requires 5-10 pumps on the gas pedal before cranking to get it started. About the same with my '65 Corvette (real Carter AFB). Both cars 'smell-up' the garage with gas fumes for a few days after they're parked. I'm told that fuel evaporation like this was not uncommon in the 'old days'; we are just not used to it with modern fuel injection and electric fuel pumps...

I installed a '70 pump you know the deeper one and do not have that problem anymore.

I have also had this problem. On my 79 Vette I changed to a Holly carb and it starts now with two pumps no matter how long it sits. I also changed the carb on my 75 Grandville to a Holly and it starts on two pumps no matter how long it sits.
On my 69 bird I have a Edelbrock carb with an electrical pump. Once the carb is primed with the electric pump it starts with two pumps no matter how long it sits. I really like the Holly carbs. I recently purchased a 65 Riviera with a dual quad setup. It has Edelbrock carbs. I have to pump the piss out of it after it sits for a long time. I am going to try adding an electric pump to prime the carbs and keep the stock mechanical pump.
Just my .02.

The vapor pressure on todays fuels is terrible and it is perfectly normal for the gas to evaporate out of the carb over a couple of days. Especially when the motor heat soaks after a drive and boils the gas out of the carb after you park it.

For those of you that put on an electric pump, did that cure your problem? I am thinking I may have a similar fuel issure myself and was considering an electric pump to prime before cranking.

I have found that if you can use the paper fuel filter in the carb inlet. then use the one with a check valve in it. That cured the problem I have seen on a few cars. You can tell the filter that has the check valve in it by looking in the inlet hole of the filter and see it. They make one for the short and long filter type.

When you do your bench test, you are leaving out an important varible. You need to have it attached to something that it 200' and will hold 200' for a number of hours. If you conduct the bench test using actual operating conditions, you will find a it to be a different answer.

Mine dries up some, but not enough that it will not start instantly.

It's impossible for it to drain back, meaning that if it drains back, you have a serious carb problem that will show up while operating the car, specifically severe load-up.

Should be easy enough to rule out a couple of things:

Evaporation : Start up the car and shut off immediately before anything gets warmed up. let it sit for a few days. If it starts right up then it must be evaporating dry when hot.

If it doesn't start, somehow try to block the fuel from draining back. There are 2 rubber check valves in the fuel pump (I'll see if I can find a pic) if they have degraded/cracked due to ethanol gas (or being 40+ yrs old) they would still allow the pump to work but would not stop the gas from slowly draining back. Its basic physics as the quadrajet carb level is above the gas tank level. If the carb float is low and needle is open in the carb seat, then you need the fuel pump check valves to hold the gas from draining back. (or a one way check valve type fuel filter as Wade suggested above)

If you have a rubber fuel line anywhere in the system, try using a vise grip to lightly pinch it off & try in a few days. If you don't have any rubber then I suppose you could undo the line from the tank at the fuel pump, plug off to the tank and monitor if its slowly dripping backwards out of the fuel pump.

It could also be the wells that were epoxied are still leaking depending on how it was done. You'd have to pull the carb off to rule out that one.

These are a few things you could try, as always be very careful with gasoline & taking apart fuel lines. We tend to forget how flammable & dangerous gas really is. My little disclaimer LOL !!

I scanned a pic of a fuel pump out of a manual I have. You can see the inlet and outlet check valves that only allow fuel to flow one way. If the rubber on these valves is not good you could have some seepage back. The pump will continue to work fine although won't be as efficient per stroke.

Easy to fix with a new fuel pump if this is in fact the problem.

I installed an electric pump as a primer pump.
I let the pump run for a few seconds, start the car and shut it off. Works perfect!!! The electric pump MUST be the type that lets the fuel flow when it's off.