His header glow is not from over advance. More probably just the opposite. But, yes, you CAN overadvance the thing under hard load IF the vacuum advance is hooked to the PORTED side as he stated his is. Think about how it works. The vacuum advance is there to help street driveability and economy at light to moderate throttle opening. At that point, the mixture volume in the cylinder is low 'cause the throttle plates are nearly closed and restrict the flow of air/fuel mixture into the cyl. Because this mixture is harder to light cause there's less of it, the burn needs to start slightly earlier. Vacuum in the manifold is high under these conditions. So, if the advance can is hooked to the MANIFOLD source, the vacuum advance adds 10*-20* at this point to the mechanical advance of say 38*@2800 rpm with light throttle opening like going slightly downhill at 60 mph. For a total of 48*-58*.
Now, when you open the throttle blades, the motor is now receiving full air/fuel volume. The amount of timing necessary to achieve burn is lessened. The manifold vacuum drops to virtually zero. The spring pressure in the vacuum diaphragm overcomes the lessened vacuum and pushes the advance plate back and now the timing is controlled for the most part by the mechanical curve.
If you hook it up to the PORTED source, the vacuum RISES as airflow thru the carb venturii increases and pulls the vacuum pot open. This will ADD the 10*-20* to the already fully advanced (say 38*@2800rpm.) to give you a total of 48*-58*.
My motors don't like that much timing under full load and I doubt yours does either. Try it with a timing tape and you'll see what I mean.
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