i have a 83 f 150 4x4 with a 300 6cyl. starts great and runs good for about 5 min.
after about 5 mins it loses power,idles rough and backfires threw carburetor
when i give it gas. after it sits for a bit it fires right up runs great for about
5mins. carb is new and has been working good. any ideas
Check the fuel pressure going to the carburator. If you have a weak fuel pump, the pressure might be insufficient to maintain pressure after the engine has run awhile. Try a heavy throttle foot as soon as the engine is fired up. If it starts losing power immediately, that's another clue. If you have good fuel pressure, try testing the PCV valve, and see if it is sticking. If the truck hasn't been tuned up in a while, some new ignition parts may be in order, and you should have the output of the coil tested.
The condition is known as "spitting" or "front-fire" through the carb. Check all components of the electrical system, expecialy the spark wires - making sure they are in good condition and not touching. Also check the distributor cap for cracks. As this occurs when the engine has warmed up, check any and all plastic-rubber components for failure which will expand after heating up.
Check for vacuum leaks
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