I have a '73 440 with a holley 4bbl that ran pretty good until recently. If I start the engine cold it will start right up and idle for about 30 seconds and then die. If I try to start it afterwards, it won't start unless I pump the gas a few times (I normally don't have to pump the gas at all to get it started so this is new). When it does start it just putters and dies again seconds later. The fuel pump is pumping fuel fine. I checked for obstructions in the fuel line. The I put a new fuel filter on hoping it would help, but it didn't. So the filter is clean. The carburetor is squirting fuel. I haven't touched the mixture screws because it was tuned fine the other day. Any thoughts? Thank you.