Is yours setup for a dummy 12 gram with the ASA coming off it on a 45 degree angle towards the trigger guard ?
If so, I've got one just like that ( I think someone locally did this conversion, since all the ones I've seen like that are on the west coast ) and it also suffered from really low velocity after I rebuilt the valve (it had been ruined by it's previous owner). It turns out that the valve spring in it was completely wrong. I swapped it out with the one in my Piranha SB and it jumped up quite a bit. So I went to a hardware store and bought a spring that's pretty close to the Piranha one and that seemed to fix it.