Duravit Stark Toilet Flush Question

[HaplessHubby]

I have a Duravit Stark 1 close coupled toilet which has begun to flush poorly. The cistern functions fine, but there seems to be a partial blockage just where the water enters the toilet pan which results in the main stream at the back of the pan being reduced. I have investigated and there seems to be a piece of rubber partially blocking the flush flow.
Does anyone have any ideas where the rubber may have come from or how to improve the flush. At this point the system is not leaking but my thinking is that the rubber may be part of a sealing gasket somewhere in the close coupled assembly. Any ideas welcome.
Thanks

 

[mogget]

I don't know that particular bog, but is it the doughnut that seals between the cistern and the pan?