The problem is with your vacuum cutoff and is not related to the Mercedes w123 Pivot Mount Accelerator Linkage, unless you bumped or separated a vacuum line while you were installing the pivot mount. You likely have a vacuum line that is leaking because it’s disconnected or one of the rubber connectors is not sealing tight. You’ll have to get a vacuum pump and troubleshoot your way through the vacuum lines. I had a line off from my heating controls, and one of my vacuum check valves was just a pass through and not holding the vacuum. I found both problems and I rebuilt my vacuum pump (which is simple, but the disassembly is very hard. You have to have the right tools, and you have to clean the bolts and drive the Allen sockets into the bolts.
Kent Bergsma, from MercedesSource.com, has a video on this subject. https://www.youtube.com/watch?v=QgWIX2DQo1k&t=392s. For beginners, it’s time consuming with lots of disassembly.