you need to find out if the single stage you have is working solely on the first stage screw, or solely on the second stage screw.
Wind the second stage (rearmost) screw in a full turn and compare - if it's basically the same, but fires earlier, you know you are solely on that screw.
put it back to where it was, and then bring in the first stage screw, progressively more, until the second stage is felt, then rrduces, then goes... then come back half a turn, job done.
If the test indicates no change, you are solely on the first stage screw... so back it out until the second stage comes in. if it doesn't and the pull just gets really long, or even never fires, then you will also need the wind the second stage in a couple of turns, and then try to find it again.
this is a start, but should get you to something useable, and then fine tune from there.
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.