The set on Strawberry Jam can sometimes be tricky. A runny jam is often down to one or more of the following: Too little sugar - sugar plays an important part in setting and preserving jam. It's all to do with pectin chains forming to make a gel. Sugar binds to the water leaving the pectin to get on with forming chains. The usual rule is equal weights of sugar and fruit. If you don't use enough sugar you can end up with a runny jam. Too little or too much acid - lemon juice is essential in strawberry jam to help the pectin chains bond. Too little lemon juice and your jam won’t set. But too much lemon juice can damage the pectin, thereby limiting the jam’s set. The amount of lemon juice in my Strawberry Jam recipe is tried and tested and gives a good set. Temperature too low - jam needs to be heated to between 104 to 105 ℃ or 220F to achieve setting point. Heat the jam up again and use a jam thermometer to check the temperature.