<VV> moveable timing

[pete]

This summer I had a very difficult time maintaining the proper timing on my 68/110/pg/. It was inconsistent and always pinging, regardless of octane and timing.During this period,I blew an exhaust valve[car missed convention,we didn't]
   Had the heads repaired,the carbs and distributor  rebuilt, and also installed a new Clark's harmonic balancer.This was a smog equipped engine. I removed the smog pump and replaced the exhausts.  The carbs and heads are the original ones[smog],although as I said they have been rebuilt.
     Once I reassembled the engine,I tried to set the timing and found it would not run properly at 12 or 14 degrees. Thinking my timing light to be faulty,I asked a fellow Corvair owner to come over to check his light against mine. My light was fine. We found that to get the car to run smoothly,it must be set to 36 to 42 degrees advance. In addition,the timing appears to change between engine start ups;i.e. from 36 degrees and 15 minutes later it will 42 degrees.
   What could be the root cause? Is something slipping internally?
           Baffled!         Pete