What would cause a 73 302 in a 66 mustang to backfire slightly at highway speeds? App. 65/70 mph. car seems to run ok other than that.
302 had new head gaskets installed by a slightly untrained mechanic(myself). Any help would be greatly appreciated.
Did this problem start right after you did the head gaskets? Backfiring is usually a result of poor or slow combustion but it can be caused by several different factors. A lean condition as FE said, could be the cause. If a vacuum or induction leak, my experience has been that it will usually get worse if at speed or going downhill, you take your foot of the gas and coast. Another example of a possible cause could be an ignition deficiency even as simple as a bad plug or lead. Good luck!