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Q:

miss firing on 1991 Ford Thunderbird

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
my engine idles in park but in gear sputters and back fires when you gas it! changed spark plugs and cap
(2) Answers
This sounds load related. There isn't any load on the engine when it's in neutral, but when you put it in gear, there is. This could be related to the ignition or fuel system, you'll need to have a diagnosis of both systems performed to know what this might be.