Stalls while setting timing

Hi all,

While I'm trying to set the timing and I'm turning the distributor and every thing is fine at about 1000 RPM & 10 BTDC.

I turn the distributor clockwise just a hair more, and the rpms drop and the engine stumble and stalls. Timing for this vehicle is supposed to be 4 ATDC with the vacuum on.

Vacuum & advance does work.

What am I doing wrong.

This is all on my 72 Mercedes 250 W114 M130.

Any help would be greatly appreciated.

Thanks