Sometimes their will only be an S or SS and not the entire "Syncro spark " scribbed into the case near the date . Brisons will also bite if they are too lean on the low needle and have the throttle open too far , or flooded (odd right!!!)
If you verify that you do have a synchro ignition I would recheck the fixed timing again and be sure you are setting with a counter clockwise rotation of the crankshaft. If all else fails to resolve the issue replace the p/u sensor I have had them cause your concern but it is rare . Usually the sensors just completely fail. The usual cause is an open high tension lead by the spark plug cap being off the plug while operating the ignition.
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