Hi everyone i need some help. I have a vw golf estate m reg, what happening is sometimes when i start the engine it is backfiring from underneath the carburetta. And there is also a smell of fumes inside that car when it happens. It ahs had a new exhaust on most recently so i know it is not that. And the timining belt is fine. MY brother-law has had a look and he wasn't sure whether if it is the valves or the spark plugs. If there is someone out there to help me i would be very grateful.