do not refit the weight. What I'd do next, using your data above as a baseline, is add a couple of washers for preload, and see what happens.
if everything goes up uniformly (I expect it will), then you have a choice of leaving as is, or going further depending on your reqs.
If output falls, or it becomes unpleasant, report back - options in this situation would include a slightly shorter (stiffer) spring, reducing piston weight, etc. (or just leave as it was)
I know Jim will disagree due to a lack of scientific control on all the possible variables (Happy New Year JT ), but the Exact/Express comparison indicates to me that you could up preload a bit and not have a slammy cycle, with a useful power uplift to around 10 FP. If you wanted nearer 11, you'd probably need a slightly stiffer (but not longer) spring, rather than spacing this one, but the "add a little preload" test (say 3 -5mm) will inform that.
Last edited by Shed tuner; 12-01-2021 at 04:55 AM.
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.