Hello!the stock was a 1377.i got a ft valve with piston and a 24" .22 barrel from mellon, and a brass breech with hollow probe from gma.I drilled the check valve, angled the poppet , slightly angled the exhaust port, cut 3 threads off the valve, and increased the exhaust port to 0.157" and barrel.I have joined because i am getting low results from it.i was getting almost same low results without the valve mods.I have also changed the oring on the piston and not much difference.5 pumps -341fps10 pumps - 472 fps15 pumps -534 fps20 pumps - 583 fps25 pumps - 593 fpsNot leaking any air through port but it seems that after 10 pumps no more air goes into the valve but pushes the handle back mostly.
Need to do a systematic check ....Say at 10 pumps ... fire gun, then Recock without pumping and fire again. ANY residual air left ?Try again at say 20.What is suspect ... lack of hammer strike energy, tho whats odd is power go's up more ya pump If a low hammer strike, as internal pressure rose within valve, it would become harder to crack open and velocity would eventually fall at some point as pumps went higher. Also odd is most times max power peaks 15/20 pumps or so.
Sounds like a clear case of an improperly adjusted piston. Make sure there is no air in the gun, then on the first pump stop the pump arm right when the effort to close the arm rises sharply, and then measure how far the end of the pump handle is from the tube.whenever you install the flat top setup, you should take o ring off the valve and then when you move the pump arm no air will go into the valve, and this allows accurate measurements. Of course, the screw that goes through grip frame into the valve MUST be in place. In this situation, the rule of thumb is 3/4" measurement, but as low as 1/2" is fine and personally my 1322 measures 5/8".since your gun is all put together, it will hold air as you pump and so the place you stop to take the measurement can vary a little.report back what your measurement is
Man, you are killing the linkage and tube at the pivots. I would guess the air is getting back out past the pump piston. When I do these mods I'm not doing them to get more speed or distance out of my gun... I do it to get the same performance with less pumping. Better efficiency. If my goal were going to be higher performance, I would absolutely need to consider the added stress I'm putting on the machanics. "I just put $200 plus in a modded gun so I could shoot it at 900 fps ..... and after 300 shots I replaced the pump linkage and tube because all the holes became oblong and my pivot pin was bent". "And Doc says that the soreness in my elbow will eventually subside with plenty of rest". I have several Sheridans and good old wood and metal Crosman pumpers. I rarely pump them over 5 times. And the job gets done.
ya know, maybe your barrel is a little too long. if I recall correctly, the optimum length is around 18". could be you don't have enough volume/pressure to use a barrel that long. I wish RSTERNE would jump in here, he's the resident authority.