pellet weight and velocity???

I've been shooting jsb 25.4 gr out of my .25 wildcat. I haven't tuned it or changed anything. So according to AOA these pellets are averaging 910 fps. Now I'm also shooting the jsb heavies 33.95 gr. and they're averaging 805 fps. My question is can I adjust the hammer spring tension to increase the velocity when I'm shooting the heavies and if so what would be a good fps ?
 
I don't at the moment but that's next on the list. Thanks for the info , so when I do get a chrony I know what range of velocity to aim for. I'm no tuner either but with the research I've been doing on this it seems pretty straightforward. Its just tightening the hammer spring allen bolt . Thanks again and if anyone with a wildcat has personally done this I would appreciate some feedback. Thanks