Yes, my problem is solved by having modified the baffle (the plate inside the muffler that looks like a Swiss cheese). I have cut away about 5mm at the top and bottom of the baffle so the exhaust flows more freely out of the muffler. I first tried to bend open the small openings in the baffle. This made the engine run a bit longer before choking, but I either had to remove the baffle all together or cut away bits of it before it would run consistently

There is of course the possibility that your problem is related to air getting into the tubing between the needle and the carb. Try changing this tubing first, if the problem persists then remove the baffle (or the other way around).

Good Luck
Henning