Carburetor going dry.

[chargerdon]

First, my experience is only with Edlebrock carbs...Their design is such that the bowls are directly below the throttle bores and that promotes evaporation.   On the Edlebrock the needles/seats are on the top of the carb and even if dirty would have NOTHING to do with evaporation.  They only function when the fuel pump is running and there is no way gas could leak out of the Edlebrock carbs thru them.
So, i do not believe that a carb rebuild or changing the fuel filters would accomplish anything other than a rebuild would include new gasket in the accelerator pump so that it pumps more. 

An electric fuel pump would help, as you can turn the key on and leave the pump run for a few seconds to fill the bowls before cranking.

I do not have any experience with other brand of carbs...   One sure fire solution in to replace your carb with a fuel injection system.. 

[torredcuda]

OP said 340 6 barrell so I assume we are talking Holley 2 barrells. My only experience is with Holleys - I know very little how Edelbrocks and Carters work.

[DeathProofCuda]

First, my experience is only with Edlebrock carbs...Their design is such that the bowls are directly below the throttle bores and that promotes evaporation.   On the Edlebrock the needles/seats are on the top of the carb and even if dirty would have NOTHING to do with evaporation.  They only function when the fuel pump is running and there is no way gas could leak out of the Edlebrock carbs thru them.
So, i do not believe that a carb rebuild or changing the fuel filters would accomplish anything other than a rebuild would include new gasket in the accelerator pump so that it pumps more. 

An electric fuel pump would help, as you can turn the key on and leave the pump run for a few seconds to fill the bowls before cranking.

I do not have any experience with other brand of carbs...   One sure fire solution in to replace your carb with a fuel injection system..

Agree with chargerdon.  Holleys are the same, with needle and seats on top of the fuel bowl, so with the engine shut off there should be no pressure to push fuel up and out of a needle/seat valve that is stuck open.  All carbs operate under atmospheric pressure, so the fuel bowls are vented to the atmosphere, unlike fuel injection systems that are under pressure.  Current gasoline blends are designed for new cars with fuel injection, so they evaporate faster under atmospheric pressure than gas did back in the day.

[DeathProofCuda]

Except for the fuel shot supplied by your accelerator pump, all of the fuel being supplied to your engine is being drawn in by vacuum through various carb passages, which is why the fuel bowl has to be vented to the atmosphere.

[rikkitik]

 "Fuel drainback" is virtually an impossible myth. Though I haven't seen "every" carburetor, I've worked on hundreds (sidedraft, updraft, downdraft), and haven't come across even one that the fuel enters the bowl below the fuel level.
Also, in pretty much every installation that was designed for a mechanical pump application, there is positive (though very slight) pressure at the fuel pump inlet. Once the "fuel siphon" is established (the draw from the tank) you will get fuel flow if you disconnect the fuel line at the pump inlet. Plus the "check valves" (usually just "rubber" flappers) in the mechanical pump prevent drainback possibility if the pump is at all functional.

[worthywads]

I've had issue with Holley accelerator pump gaskets leaking.  Replaced the with ethanol rated gaskets and eliminated the recurring leak.  Should be obvious evidence of this gasket leaking though.