[Big-Foot]

Nick - The bolt in the linkage trick is not really helping you, although it does give more of a kick in the pants feel.
Here is the reason you get the kick feeling;
When you are opening up the throttle gradually, the carburetor's circuits are all interacting with the task of enriching the fuel mixture to handle the increase in air. You transition from Idle to main-metering circuits through the Acceleration Circuit (accelerator pump) - your power circuit (power valve) helps by enrichening the main metering system to cover-up any flat spots after the acceleration circuit drops out. When you open the throttle far enough to engage the bolt in your linkage, you force the secondaries open early which causes a momentary "lean" condition (slight to major bog) - the carb's secondary main metering circuit fights to start flowing fuel to richen the fuel mixture back up - suddenly it is flowing fuel and VIOLA you get the kick in the pants that you did not feel before!
Measuring the acceleration with a stop watch and distance between a properly tuned vac secondary carb and one that has the "bolt" trick, will always show that the properly tuned carb will win. Tom is right - keep working with the springs, squirters and accelerator pump cams and you will have superior performance.