I have a 73 f-250 2wd with a 360 2bbl. My truck will not idle and keeps backfiring. I have checked all vacuum lines, alternator, battery, spark plug wires. I can't figure it out. It just started doing this out of the blue. The truck will run fine at high rpm but as it comes down it just shuts off. The carb makes a loud hissing noise sssssssssssss. As I give it gas it will backfire spit and sputter and will eventually get up to rpm. Please help

 

Your float needle in the carb may have a piece of dirt in it, making it flood out. Right AFTER it stalls, look down the carb throat and see if there is fuel dripping down into the engine. If this is the problem, the backfiring is being caused by raw fuel in the exhaust system.

 

The hissing noise at the carb is a vac. leak..Which causes it not to idle and to backfire..It runs at higher rpm cuz it is getting more gas to over come the added air intake..Check the base of the carb., the gasket, and spacer if you have one..If this doesn't do it, come over to the FE forum..