mxhideout: Not a carb. expert, but my understanding is that they are 100% vacuum operated, on our vintage engines at least. Just so were communicating correctly, the rear two barrels are the secondaries, the front two are the primaries. The primaries open by the throttle lever via throttle cable connected to the carb. The secondaries open by vacuum when the engines develops enough low pressure at speed/load, somewhere above 3000 rpm or so (seen several opinions on when).
thatsmrmastercraft: Understood, mine are working now. I was just looking for some of that 'free horsepower' he mentioned!
1988 Tristar 190 Sport, 351W w/GT-40's, 1:1 Velvetdrive