mustang wont stay running when put in gear

Is your check engine light on? sputtering could be a number of things, including the type of gas you're using (which wont trigger a check engine light), try to use some SeaFoam (its like $7 - $8) in your gas tank next time... it will have a trail of white smoke for the first few minutes of use, but it should clear out a good portion of the system.. you can pour it in your gas tank, oil and even let your vacuum lines suck it up.. give that a try first and see how that treats your Mustang.

I have a 1983 3.8l mustang just rebuilt the motor and new carb. still wont run in gear