Pinging
this kinda covers the vac pot
http://www.sweptline.com/tech/engine2.html
What seems to be the problem using the light? Is it jumpy? I'ved used a cheap harbor freight adj timing light before. Or i just run timing tape on my balancer. You need to find the total timing, and when it comes in. Could be vacuum related. you can plug off the vac line and drive it around. see what happens