Disassemble the valve and stack another one or two on the one inside there, you will have to remove the sleeve to do this and can't put it back in. Doing this will make the rifle velocity more stable, it can rob a few FPS but if stability is needed thats how i do it.
I was able to detune the rifle somewhat by adding some washers on the back side of part #200, it's the block the hold down stud screws into. The stroke on the valve is now about 1/32 of an inch and has the rifle shooting in the high 500's. I lost count of the shots on this fill but it's been a bunch, when it emptys out I'll finish the work on it and see what it's shooting. The deviation from the average shots is around 1 to 2 fps with out of the box pellets so it will be consistant. The groups I've shot out side with a light breeze has been around 1/2 inch at 20 yards using the diopter sight on the rifle, get the barrel broke in and a good shot behind the trigger and it will be fun to see how accurate this thing is.