The mainspring is almost never the source of a heavy trigger pull. Do not try to reduce the mainsprings pressure.
The source is usually due to one of three things.
If the full cock notch in the tumbler is not machined at the correct angle with respect to the axis of the tumbler's shaft, the nose of the sear will be forced to climb uphill as it is withdrawn from the full cock notch. This condition will result in the hammer or cock appearing to move slightly to the rear as the trigger is pulled. This happens just before the hammer or cock is released.
If the lock's full cock notch is wrong, have a good gunsmith modify it. If you try to do this yourself there is a good chance of overdoing it. This can result in the shape pushing the nose of the sear out of engagement which will cause the gun to fire even without anything pulling the trigger.
The second thing that can cause a heavy trigger pull is, the location of the pin which the trigger rotates on is too far from the arm on the sear. This is a matter of leverage so, moving the pivot point closer to the sear arm (usually moving it rearward towards the butt of the gun) or, bending the sear arm so it makes contact with the trigger blade is the only fix. Bending the arm on the sear has it's risks. The nose of the sear is hardened and somewhat brittle. On some guns, the entire sear including the arm is hardened. If the arm is hardened, it will break if it is bent. To see if bending the arm is possible, use a file to try to remove a little material from the arm of the sear. (It is the thing that sticks out at 90 degrees from the lock plate. Moving it up, releases the tumbler.) If it is soft and files easily, it is OK to bend it forward a bit to make it closer to the trigger pivot pin.
The third common source of a heavy trigger pull is due to the sear spring. It is usually a small U shaped spring at the rear of the lock (TC and Lyman use coil springs). You can reduce the force created by the sear spring by filing the side of the (usually) lower leaf of the spring. It is the part of the spring that is pushing directly on the sear. Reduce the width of this spring, tapering it from the area of the bend, where it's width should be left alone, toward the tip of the leaf that contacts the sear. The taper can reduce the width from the original width at the bend, down to about 3/32" wide at the tip that contacts the sear. Don't overdo this. The spring must have enough power to always engage the nose of the sear with the half cock notch and the full cock notch.
Do NOT try to reduce the thickness of the leaf. Making the leaf thin will usually cause it to break.
