I have a 1974 Ford Econoline E200 Van with an 8 cylinder with the present symptoms: It starts and runs great, but once is warmed up, it feels like it is not injecting fuel. It loses power and slows down, or dies ... especially if I drive it uphill. I have to wait 5 minutes to get it to start again. I have replaced the starting coil, gas filter, and the carburetor, and it still is doing the same thing. The gasoline pump seems like is working fine, but ... I need advice. What could be wrong with it?
If you look at your engine you will see that in 1974 almost all emission controls were connected to or controlled by a vacuum hose. As the hose ages it cracks and causes vacuum leaks. Many of the components had no vacuum applied until the engine was warm and above idle speed. This can result in good idle and performance cold but a vacuum leak and poor performance when warm. I would start with a close visual inspection, paying particular attention at connections and bends. Check the hoses on the thermostat cover, choke pull off, and the aircleaner snorkel. You can try to disconnect and plug with a golf tee the hoses one at a time and check for improvement. Another old trick was to apply a light throttle while an assistant moves an unlit (but flowing) propane torch near the connections and listen for changes in idle speed. The propane will slightly correct for the lean condition caused by the vacuum leak. Obviously use care if you are doing this in a garage or have any reason to suspect a stray spark.