As you continue to go stiffer and stiffer on the main spring it can actually richen the entire bottom end and mid range until the high speed opens. There is a delicate balance between getting your main spring and jet right and your barrel valve set properly.
Sometimes, some nozzles don't always spray very well at low rpm which tends to hurt throttle response. The only way to tell this is to flow your nozzles or fuel system.....as it is extremely difficult to see the nozzle spray when the engine is running. ;-)
If you have weak fuel pump that does not put enough pressure behind the nozzles to make them spray good, or if you cannot get the barrel valve adjusted to elimate a lean stumble, the next step would be to go to the next stiffer spring. Kinsler usually rates their springs in wire diameter, and they actually make several different ones. You can easily measure it with a dial calipers.
The first problem is to try and figure out the source of the problem you are trying to fix. If it is a stumble.....it could be a lean stumble or a rich stumble....the best way to tell which one you have is to try and use the fuel shut off valve to manipulate the stumble. If you close the valve part way and the stumble goes away or is not as bad....you are too rich on the main.....if you close the valve and it gets worse or stays the same....you may very well have a lean stumble............................and sometimes all the tuning in the world won't get rid of the stumble and it's time to take the drag race cam out and put something in that wins races and takes off.......or get a clutch or a stall converter!
I hope that might help answer your question.
Mark Burch, Lincoln NE
Rome wasn't built in a day......but they sure didn't
waste any time burning it down!