Every time I start my car in the garage ( with the door open and the rear of the car at the edge) when I'm done I smell like exhaust / gassy smell! The plugs look light tan and the car runs good, except for a slight stumble at idle that just happened. Do you think a O2 sensor/tuner would help? I have no clue what the carb settings except for Vac are. What are some good ones?