Thanks a lot brutus for your quick, informative and "balanced" reply..!!!! Since the engine ran well enough even before my – “just in case” - dismantling of the carb and since I found very little dirt inside I will allow the old cup to fly a few more times. The metal screen filter had very little debris in it.
Now, please let me "pick your brain(s)" on one more item regarding this carb: Previously the engine has been running with cylinder pointing upwards but my Bucker Jungmann 1/4 scale requires me to invert the engine not to destroy its scale looks. My understanding is that the small hole in the quadratic pump cover(see attachment) should always be placed with the open hole pointing downwards in order to drain any excess fuel “outside” of the membrane so I must turn the cover accordingly..?!?!

Please correct me if I´m wrong here. My manual says nothing about this.