when i start my car and my dad is in the engine bay mucking round with the carby it wont run unless he has his finger down the throat of the carb and holding the piston up when he lets go the car dies and it kind of sounds as though it surges when it is running

what is wrong with my car????????????????

Id screw the mixture screw down a bit and see if you can get it to run without having to hold the piston up (which causes more fuel to be released = richer)