I would connect an external vacuum gauge, great tool for many items that need tweaking on any motor. With the vac gauge connected, first you can see exactly how much vac you have, 16-20 range is good, you then can fine tune your timing, rotate dist to the highest reading. Adjust the carb mixture via the vac gauge. If the needle on the gauge is bouncing all over the place you could have valves out of adjustment, etc.
Best of luck
