Just hooked up a boost guage (also measures vacuum) to my manifold and at idol after the car has warmed up the vacuume fluxuates rapidly between 30 and 50 cm Hg I didnt get a chance to drive it with the guage hooked up (didnt have a long enough hose, will do it tomorow) but maybe because the way the vacuume is taken from the manifold (alternating throats) the signal is fluxuating too much for the modulator to pick up, Just a thought.