Vacume Modulator??

Hey i have a question about my c4. Its a pre 70's c4 with mild valve body work and a 3000rpm stall torque converter. When i am just taking off easy and i manually shift it from 1st to 2nd it shifts right away with no lag, from 2nd to 3rd it has slight bit of lag but still shifts on demand. When i am out at the track and racing and i shift from 1st to 2nd it shifts on a fly but when i go from 2nd to 3rd it doesnt shift at all until i let off the gas and then nail it again. It has no kickdown linkage hooked up to the carburator but the kickdown arm on the tranny is moved halfway forward and tied. Now i was wonder if the vacume modulator needs to be replaced or adjusted, and if it needs to be adjusted how much should i turn the screw one way. thanks :smile: