1966 Ford F-250 backfire

Check your ignition switch it may be bad causing the running after off and check wires near starter relay youmay have one in the worn position. For back fire check for a cracked dist cap or coil

I DON'T KNOW THE TUNE UP SPECS FOR YOUR RIG

IF YOU HAVE IT SORTA TUNED UP PROPERLY IT SHOULD IDLE AT THE CORRECT IDLE SPEED.

MY BEST-US GUESS IS YOUR IDLE SPEED SHOULD PROBABLY BE SOMEWHERE BETWEEN 700 - 800 RPMs

IF WHAT YOU "MEAN" BY IT CONTINUES TO RUN WITH THE KEY OFF, IS IN ALL ACTUALITY SPITTING AND SPUTTERING THEN FINALLY DYING, WITH THE OCCASIONAL BACKFIRE (SOME FOLKS REFER TO THIS AS "DIESELING")

IF YOUR IDLE SPEED IS HIGHER THAN IT SHOULD BE (EXAMPLE, MAYBE 1200 RPM), IT CAN DO EXACTLY WHAT I DESCRIBED IN THE PREVIOUS PARAGRAPH

IF YOU BACK OFF ON THE IDLE SPEED AND IT WON'T RUN (IDLE), YOU NEED TO DO SOME CATCHING UP ON YOUR TUNING AND MAYBE LOOK FOR A POSSIBLE VACUUM LEAK, IF ONE EXISTS

LET US KNOW HOW IT GOES

THE MEDIC