We've been having a problem with our Ultimaker 2. The print quality in general seems very good, but the infill is very bad. We've found by setting the flow-rate much higher (as much as 140%) we can mitigate the problem somewhat, but it negatively affects the rest of the print.
The images below are of models printed at normal quality (Cura), but infill set at 25%. We are using ABS.
I've tried to show the infill from the side to give an idea of how insufficient it is. Note, however, that the rest of the print is not bad at all.
Sometimes entire holes will fall in because of the lack of infill, as shown here.
We've tried many things already to fix this. I've used the "atomic method" to clean the nozzle; I've opened up the entire print head and cleaned it out (soaked it in acetone over the weekend). We've checked the feeder and Bowden tube.
Any advice on what I can investigate, or what could be the problem, would be greatly appreciated!