This has been a while and I want to give an update. After sending in my engine to Heidentuning and getting it back with a "all good" reply and a few days of running "ok" and after that another summer wasted on a engine that won't run properly (and never has), I discovered XS-Laden in Bavaria and sent my bike there in autumn 2017. When they opened up the engine, they discovered a total mess. The basic problem are the way to thin cylinder sleeves of the 750ccm bigbore kit. A solid 750ccm is not possible without modifications on the case. 6 months later I finally got a really powerful engine with custom cylinder sleeves and totally rebuild engine from XS-Laden and the engine is finally running how it was supposed to from the start. Boy what a difference! Since then I put around 3.000 km on it, no problems at all. So in the end I wasted thousands of euros when trying to find out, why the engine was never running properly (Exchanging carbs, exhaust system, etc.). I can't speak for other people out there who bought their 750 bigbore from Heidentuning, but getting a "all good, no problems" reply AFTER sending an obviously destroyed engine back to the builder is really beyond my understanding. I also attached some pictures of XS-Laden, when they opened up the engine.