[Bellerophon]

Hah, looks like we independently came up with the same adjustable power modification. That little shelf under the spring block is just the perfect place to put a nut and screw for power adjustment: only differences are that I have the screw head facing outward so I can adjust it with a screwdriver, and also I just cut a chunk out of the midsection of the spring block and JB welded the two ends back together rather than cobbling a replacement together.
As for the power problem on single action, is there anything inside the whole hammer-strut assembly that the hammer spring can catch on? In single action the hammer goes back farther, I figure the spring might catch a protruding edge and compress against the edge rather than the top of the spring block.