Hi,
The mainspring is very weak and needs to be strengthened or replaced. I made new springs for it when I've used these locks (rarely). Look closely at your photos. On the original lock, note how close the flint and top jaws overhang the middle of the pan. On the Davis Twigg, the flint is left hanging well above the pan. Unless you use very long flints, it will barely kick the frizzen over and sparks fall everywhere with only a few going into the pan. As a result, after a few shots when the flint is duller and fouling has built up, the lock fails to produce sufficient sparks to ignite priming reliably. The flintcock is too tall and needs to be bent into shape, but better, replaced with a smaller flintcock. I replace them with TRS series 569 flintcocks, which work and look much better. The big flintcock that comes with the lock looks really clumsy and out of proportion. The flintcock is also too massive for the mainspring so the smaller cock helps that as well. However, I make new springs that are shorter so the lower leaf is shorter and more stout. After doing all that and balancing the frizzen spring, you can produce a pretty good lock but the real issue is the tumbler hole is too far back. It should be 1/8-5/32" further toward the pan. The boss for the upper bridle screw should almost touch the rear of the lock plate bolster.
dave
