Ford Repair: Vehicle backfires, ford econoline, vacuum leak

Question
I have a 1984 Ford econoline 250 that backfires not everytime I shut off the engine, but every other time. I have no clue what the problem could be. Someone said it could be the timing. Need your help.

Answer
have timing checked-if it is idling to fast that will do it also-try leaving it in gear a few times while you shut it off if it doesn't backfire then it may be as simple as an idling problem-bad carb dashpot or vacuum leak will cause it also. good luck.