Bring a saucepan of water to a simmer and cook the quince together with 200g sugar for 40 minutes, until quince are very soft. Pour into a food processor and pulse until smooth. Allow to cool then taste test and add more sugar if desired. Stir in pear liqueur, place quince puree into the fridge and chill for at least 4 hours. It has to be very cold before you put it into the ice creram machine.
Pour mixture into the ice cream maker and churn according to the manufacturer's instructions. Transfer into a freezer container and freeze. Take out the sorbet 10 minutes before serving. Otherwise it will be too hard to scoop.