Low vacuum can be attributed to leaky lines and gaskets that admit air into the manifold other than through the carburetor venturies. It is also possible with a "big" cam. If cam duration is 230 deg. or more at .050 lift, you could see secondary opening issues.
Check to see that there are no vacuum leaks. If there are any, eliminate them.
To reduce the tension on the secondary spring (to make secondary open quicker), turn the secondary spring tension screw clockwise. The secondary spring tension screw is located on the air horn directly behind the choke tower. It lies on a horizontal plane.
Suggest at least a temporary installation of a vacuum gauge so that you can have more objective data for both understanding what the carb is doing now and to evaluate changes that you make.
