very rough idle

  The engine did run smoothly for the first two hours after the initial mech sync.    I didn't do the pneumatic carb sync right after that because I had just ordered a Carb Mate and it didn't come in time before the engine began to run rough with gas  overflow in the vents on both carbs.  Then I installed the carb overhaul kit and it seemed to fix the left carb but not the right.  I will look at the carbs again and especially the float needle height setting.  The right carb float needle height may need adjusting.  I'll also weigh the floats during that dissassembly.

I'm wondering about the ignition spark.  Is there such a thing as a "weak" spark issue or is it either working or not working?  That is, if there is any spark present is that good enough and only if there is no spark at all is something failed and needs replacing? If there is such a thing as a "weak" spark, what is usually the culprit to look for and can it be measured to determine its condition?

I would pull both carbs and check everything inside including the float height armature. Make sure needle clips are in the right positions.

Sounds like a carb issue. Overflowing vent chamber is an indication of flooding  I would weigh the floats first, the bowls should be about 1/2 full when removed and check for any debris, check that the float needle is sealing properly,check condition of the float chamber gasket, then do a pneumatic balance.   

Sam, Your suggestion is right on but I'm having trouble finding what weight the floats are supposed to come in at.  I looked in the service bulletins and heavy maintenance manual but couldn't find it.  Do you have the number handy or know where it is published?

I concur with Sam. I had a problem with some debris in the float needle valve that prevented the viton tip from sealing and shutting off the fuel.

In answer to your other question about a 'weak spark', yes you can get a weak spark but it is unlikely that you have a sufficiently weak spark on both ignition systems to give you the symptoms that you have. That said, ISTR that the early 912 engines had a common ground connection on the ignition generator coils that was replaced by 2 separate ones on later engines. I'm sure that there is an SB detailing this mod.