Erratic timing during vacuum advance

nm9stheham said

More likely a carb or fuel delivery issue then. Could also be a weak ignition.

Was the engine well warmed up? And does the carb have a choke? It could simply not be set right.

How far open are the idle mixture screws on the carb? Is the idle very responsive to changes in idle mixture screw adjustments?

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I would defer any carb adjustments to someone more familiar than I am. I know how to set float levels and that's about it.

Could the timing being all over the place during advance be because the engine is starving for fuel and thus reducing the amount of vacuum it produces thus making the advance fluctuate?