I have a '73 240z that I have just converted from the flat top carbs, to Ztherapy S.U.s and after I drive the car for about 20 minutes, it will backfire, and run a little while after I have turned the key off I think it is called Diesel-ing. I had the same problem with the flat tops, and was hoping that the S.U.s would fix it. Any help would be appreciated.