My recently acquired 914 seems to lose fuel pressure after running for a while. Here is what I know for sure. I have pulled out the screen in the tank and have installed a carter fuel pump and all new lines including through the tunnel and under the tank. The car is a 76 with 44IDF carbs and a holley fuel pressure regulator with a guage installed after the regulator. When cold the fuel pressure pumps right up to 3-3 1/2 psi. Letting the car run in the garage for 20-30 minutes the pressure guage drops to 0. I have installed 2 different pump and the problem persists. Any ideas.

Is the tank vent letting air in as the fuel is being used. Sounds like it's not and after running a while, a vacuum is created in the tank. Does a bunch of air get sucked into the tank when you open the filler cap? Either that or clogged fuel filter maybe?

Tank does have the expansion tank in place and blowing into the hoses it does appear to be ventilated properly and have new filter installed. Fuel pressure regulator operates properly when fuel pressure is present (cold). Last drive emptied the carbs and had to allow carbs to refill to limp it back home.

Is the pump under the tank or in the stock location? Pumps push better than they suck. I've seen fuel line collapse and suck closed on a long run under vacuum.

Oops - my bad. On a 76 the stock location is under tank. Isn't the carter internally regulated at 3 1/2 psi. Check for a kinked line.

The fuel pump is supposed to be internally regulated at 4 psi. I have had the pump in both locations in the trunk and in the original locaton under the tank. Maybe you are correct and a stiffer fuel line would fix it.

Venting problems like Motorvated said are the most common. Did you try running it with the gas cap very loose or off. Once you ruled that out you likely have a supply problem like a kinked or collapsed line. Potentially even some gunk in tank intermittently clogging tank outlet.

On a side note the Weber can handle 4 psi. Much more than that and you run the risk of lifting the needle off the seat. Bad things happen then. If all else fails, you can eliminate your regulator for a short test to see if that's your problem.