ok, you are saying when the carbs are in sync, the throttle plates are too far open. You opened one before the idle got too high.
So, barring vacuum leaks the front throttle plate is sticking too far open. In order to sync, you then open the rear.
Unusual problem, but I think I understand.
If in your shoes, with no vacuum leaks, I would pull the carbs. I would look for mechanical obstruction to closing in the front carb. Gummy buildup, piece of dirt, throttle plate improperly adjusted on the throttle shaft, etc.
edit- before doing any of this, you may want to compression check both cylinders. Either cylinder poor could cause this.
Last edited by wmsonta; 07-29-2013 at 08:26 PM.