Okay so I have a 89 firebird I did a engine swap I have a 350 vortec all stock with a 625cfm street demon carb I have about 4500 miles on it and its been sputtering when I press on the gas so I had Just copper ac delco plugs in there and needed to do a plug change anyways so the guy at the counter said I should get the platinum plugs since its carbed I put them in it stopped doing it for about a day and now its doing it again my next step is going to change the fuel filter to see if its clogged and go from there can you guys let me know what you think!!!

 

Have you considered parts further up the ignition chain? Wires, cap, rotor, coil?