This almost sounds like the secondaries are hanging up and not moving smoothly. Perhaps this is why it is now working better...it is 'wearing in' a snagged part. When you change the spring next time, disconnect the rod off of the secondaries and see if they move smoothly and easily. Also, take the spring pod off the side fo the carb and use a hose to apply some vacuum (by mouth is fine) and see if the rod moves smoothly and in prportion to the level of vacuum once it starts to move. Finally, check the vacuum port on the side of the carb where the vacuum pod mounts for free ari flow into the manifold below the carb; you can just blow in by mouth with a hose. And see that the seal between the carb and pod is good.