OK well my car just start sputtering sometimes when ever i give it some gas. I noticed it smoking a little whenever i got my test pipes and down pips put about a month ago. They smoke is not alot but i can see it sometimes out my rear view mirror. It smokes mainly whenever i let off the gas rather than when i give it gas. It just started making a popping noise, like it is back firing. I just changed the spark plugs about 2 weeks ago. If my timing was off could this cause this? Let me know what you think.. The car only has 86,000 miles. Thanks