Carburetor hissing after ignition turned off. Not sure what this is but there’s a loud hissing noise that will sometime blow off a vacuum plug. Think Gas may be igniting in the intake. 86 bronco 4.9 2 barrel carb with offenhauser intake.

 

Carefully look down the carb with a flashlight after you turn the engine off. If you see fuel dripping into the engine, the float needle in the carb is stuck open. Most common cause is dirt under the needle.

 

Could the motor be dieseling?
If the idle is to high when you turn off the motor it may want to keep running and get that backfire & hissing.
Dave ----