86 150: Not enough screw to adjust advance timing

[rickclark]

Finishing last stages of tune up with static timing check until I can find my timing light or get another. Retard and pick up timing need no adjustment. However, however when checking full advance timing, I am a long way away from being able to align mark on flywheel with mark on magneto plate. Need 15 - 20 mm more on screw.
Does this mean something else wrong? Can I replace with longer adjustment screw in order to make adjustment? What's the danger of running this motor without the advance timing set correctly? I never run at WOT.