well its a 86 chevy crew cab 3+3 c20 with the 454 with 4.10 gears 70.000
miles it keeps back fireing when you shut it off it make one big pop im not sure whats the deal.any one think they know what it could be i hope its not a timeing chain

 

Check your timing. It sounds like it is over advanced. Also if it is a carb model having the idle plates opened to far will aggrevate this condition.

 

thanks for the help ill let you know