<VV> PG Shifting

[Bill Kronen]

With all the recent conversation regarding PG's, I've got a question.  
My '66 Canadian Coupe has a 140 engine with all 4 carbs operative.  The 
PG was a change at sometime in its past from a 4 speed and the engine 
may not be original.   Shifting occurs at 10-15 mph, which seems low to 
me.  Is it and if so, can that be changed?  It works fine and there is 
quite a boost when the secondaries come on line after flooring the 
accelerator above 60 mph.  Almost feels like an afterburner kicking in. 
Neat!  Any help/info appreciated.
Bill Kronen