Well my car has been getting fixed for about a year and not sure if the body shop people have been starting my car everyday. I just got it back, and it looks great. They suggested that I need a tune up and new spark plugs. It was running kinda rough. Well I got it back, put gas treatment and oil treament and fresh gas. I started it up today, and it was hard starting it. I reved it up to 3000rpm and it back fired. Could the hard starting be because of the spark plugs? What about the back fire?

Thanks,
Jack