Marvin,
I put a new spring (original style) in my Bisley, because one of the ears was broken off. Even with spacers under the screw, the trigger pull is still very stiff (I don't have a guage). Do you find that Bisleys have a harder pull than SAA's? When I get time, I may have to lighten mine.
I found that the original Mainprings are definelty stronger in the Bisley Revolvers. Having shot in CAS, most of the guys had very light Mainsprings that once in a while would not fire a round. These were not my "cup of tea" as I feel they were dangerous when used with a very light Trigger pull. I think CAS subsequently did have a rule that a minimum weight was called for on the Trigger Spring, but not sure about the Hammer pull weight. I do have one 1st Gen revolver with a very heavy Mainspring and I have measured the Spring against other 1st Gen Springs and the measurement are the same. Even the curviture of the Spring is the same. I am at a loss unless it may be the hardness of the steel. I don't have access to a Rockwell Hardness Tester anymore.
The Mainspring deals more with the Hammer pull, but
does have an effect with the Trigger Sear in contact with the Hammer notch. My one and only 2nd Gen revolver had this problem. The Mainspring pull was fine, but the trigger pull was very heavy. I got my Kuhnhousen's Manual and read what Jim Martin had written on how the Sear and Hammer notch had to be fitted. I followed the instructions and now have about a 5 lb. pull which is fine with me, and I think within Colts tolerance for pull. The Sear has a lot to do with the Trigger pull. I use Colt Bolt/Trigger Springs only. The Wolff Springs are good, but I don't like the light pull weight of them. When I can find Colt Springs, I buy them and keep spares. Springs do break sometime, but Colt Springs seem to be the best. I think I am Old Fashioned and like things old like me, LOL!
With the SAA revolver, you need to go back to the beginning of the cocking action and work forward from there. Otherwise, you may do something that will effect other parts. In order to tune a SAA revolver action, you must check each part in the "train" of movements to find the problem. This is according to Jim martin who wrote these instructions in Kuhnhousen's manual.