What is happening to your carb is you still may have the stock intake on the motor. The stock intake is cast off set so that it works perfect for the gurtner. The carb is hitting your swing arm when it variates down. If you run a Del 15mm you need to also switch to an aftermarket center cast intake that keeps the 15mm in the center so It wont hit the swing arm and get wrestled off.
The same thing happens to gurtner carbs if you try to run them on an aftermarket center cast intake, the carb rubs and gets kicked off.
This is a unshakeable rule people. I have spoken.
In the picture the stock Peugeot intake is on the right and you can see the offset. The one on the left is aftermarket Malossi and sits dead center.