Lasagna, I have run into more problems while trying to duplicate your spring replacement!
3.25" seems to be too small of a spring size, as this does not cause the spring to push the plunger tube into the "forward" position, which is needed in order to pull the bolt back, by grace of the forward bolt lock that ensures that the breach is closed before firing (the only useful lock I keep in my bolt sled blaster). How did you remedy this problem? I am thinking of filling up the space the plunger tube has to move, but do not want to do anything more until I hear how you made such a small spring work. The stock spring is 4" in length, which is just more than space available in its seating, thus causing pressure forward on the plunger tube when the blaster is not primed. I also tried to fill in the dead space before the spring with various spacers (coins, paper, etc), but found that filling the deadspace from behind led to their not being enough room for the fully compressed spring. Enlighten me on how this "magic" 3.25" length of spring works!
Well first, let's make sure that you are using the proper spring. I was mislead into thinking that the K11s were right. INCORRECT. RW11 springs are where they are at. Just call a tool and die spring supplier in town. I have gotten mine from Ajax springs in Grand Rapids. Their website is http://www.ajaxspring.com/.
That will give you a clue as to what type of company to look for.
But, for the RW-11, 3.25 is the length that I used. Longer length springs were too long when compressed. They don't bounce around, you can hear that in the video of the test fire.
Other than that, I dunno what to tell ya. I'm sharing what I found to work, and work well indeed.