I have a friend with a 3W-140 in a 40% 3W Extra. He had the same problems you are having. He jumped through all the hoops just like you and even rebuilt the original carb. It seemed better the first flight after the rebuild, then the same old thing. Then he replaced the carb with a new one. Just like magic the problems went away. It was a long hard fought journey that turned out to be a simple fix. BTW, the dead stick landings became second nature to him. Fortunately I have had very good experience with all my 3W motors and hope it continues. I don't remember the cost of the carb. If possible you might try to borrow one just to see if it cures the problem.