I would guess that probably 3 of every 4 GM that I have worked on tune ups over span of 55-60 years have had sticky mech advance systems--timing wouldn't advance or maybe not retard-timing gets reset and then is maybe negative 30 degrees or advanced up to 60-------- causes lots of other symphonies as people mess with adjustments--------

But-----if anybody is working on any tune up---it is absolutely vital that you verify that the timing advances and retards correctly and is set properly!!!! And don't look down the carb while cranking the engine--------


Typing while your posting---------