Timing question

[68gmsee]

Re. your first question. Yes, it can start to slow down on you when advanced too much. At least it does it on mine. Also check your total advance..

And the full vacuum port would be fine. I've tried it both ways on my trucks and always end up with full vacuum advance.

Now as for the idling rough. Check your carb idle screw setting, all the vacuum lines for any leaks and also gaskets and any other vacuum area like the brake booster. Maybe start by disconnectng and plugging all the vacuum lines one at a time to eliminate them.

Also, check carefully for flooding (dark gray to black smoke). If all of that
checks out, you may be looking at internal engine problems like valves or cam.