273 Can't get too much timing - confused

I have a 273 that won't ping or kick back the starter regardless of timing.

I just put a rebuilt distributor in it, but it was doing this before. The po said the cam is original but stronger valve springs were installed.

At this point I'm at 20* initial. Ground straps on the plugs are clean all the way to the bottom of the strap. But it's not pinging or kicking back on the starter. Should I back it off? Maybe the mufflers keep me from hearing the pinging??? Or, my dang bad ears. Drove it to work earlier this week up a good sized mountain and it pulled all the way up.

Any thoughts?