Make sure that your vacuum advance mechanism works freely. Quite often the linkage is bent or misinstalled. You must remember that people have been monkeying around with this thing for over 50 years. If you can freely advance the distributer by pushing the plunger into the chamber check for a vacuum leak. The old fashioned way of doing this is taking the distributor out. You have to do this anyway to replace the diaphragm. Suck on it to produce a vacuum. Stick your tongue in front of the hole. If it holds vacuum, it's ok. If not it will leak off or not build vacuum. I've been doing it this way for 50 years and it works.