with a vacuum secondary carb if you can feel the secondaries open you have messed up. what is happening is they are opening too fast as compared to the flowing over the tube in the venturies. what happens is there isn't enough vacuum over it to pull fuel up and out of the boosters yet and you feel the motor studder until there is enough air speed to start the fuel up an out. to tune them use the lightest spring and keep going to heavier ones until the studder stops.