I recently bought a 90 5.0 mustang conv, the cars has been sitting since 2006. The guy i bought it off of said it had close to a full tank and that he went and filled it. I drove the car for a day and the following day replaced the spark plugs. The num 4 cylinder was the only one that was partially burned and minor amount of oil on it. did a oil change also and afterwards the car drove fine. Turned it off drove it a hour later and it didnt want to go when i applied the gas and it would backfire and sputter. Took it home checked the plug and they werent gapped right so i replaced the plugs again and its a lil worse and wont stay running. i heard it could be the gas, what else could it be?

 

try a new fuel filter