Generally, dark roasts contain surface oil that clogs up the coffee machine. This clogging can lead to a series of issues. Either the oil coating the machine's insides will turn rancid over time, ruining the flavor of subsequent batches, or the clogging of the device will cause the motor to burn out. Thus it is not advisable to use dark roast coffee beans in your Jura machine. There are, however, two alternative strategies that you can try using. Firstly, consider using a medium-dark roast as this can still provide a lot of the same bold flavor as a dark roast with significantly less oil. Alternatively, you can use a dark roast but with a lot of caution. Firstly, make sure that the roast is very fresh, as a new dark roast will have less surface oil than one roasted a while ago. Secondly, you should try feeling the beans if possible to check if they are relatively dry or greasy. If they feel slippery, stay away from using them. If, however, they are relatively dry, you might be able to use them in your machine. However, this strategy is not risk-free.