When you take the lid off it has a lower air pressure so it will burn easier. How much smoke are you getting on startup. Fuel is a huge factor with these things. If I mix to much oil in with the gas I have a lot of problems starting it. Or if you put gas in by mistake it won't burn very good. First thing to try is drain the tank into a can and put fresh gas in it and try again. Second thing is cjack you air bleed screw on the back. If your moped is barely running turn the screw clockwise until flames start coming. When the cone starts glowing back screw off until the flames aren't coming around the cone anymore and that should be about optimum setting.