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I prefer running the initial at 20 with no vacuum advance and having the mechanical bring in 14 to 18...

I never liked how the vacuum advance seemed to destabilize the idle, its like your always waiting for things to smooth out. Without it they always stay smooth.. Then there's that weird skip-miss-stumble sound the exhaust makes from too much timing at idle...

Have you greased the universal joints lately? Turbo 400s do go into gear hard.. Try lowering the idle, you can run it down pretty far without the vacuum advance connected..