Stock superbolt valve? Iirc, it could use some drilling to match the port in the body.
How about the bolt? Try changing the bolt o-rings if they are original, many were too small and useless.
Does the barrel screw all the way in? Some won't go in that last turn, which makes a gap and screws up the ball seating and the bolt seal; you loose a lot of air out of the feed neck (in addition to the air lost from the bolt o-rings).
These are items related to low velocity for a given pressure. Farting is the balance between your springs, hammer weight and pressure at the valve. You can have good velocity with farting. If you have the things above under control, you may be running too low of a pressure considering the volume of air in the body and the response of your regulator. If you don't have enough of a reservoir of air behind the valve for a given set pressure, the regulator has to respond during the shot, which is usually too late. There's not enough residual pressure to hold the valve shut against the hammer bounce (if there was in the first place).
The valve spring and the pressure help hold the valve closed; more means less farting.
The main spring helps open the valve, storing energy in the hammer. A lighter main spring (hammer spring) is less farting. Lighter hammers mean less farting. Keep in mind that turning in the IVG is preloading the main spring. This puts the main spring more in play when the valve is closed. If the IVG and main spring length are such that the hammer doesn't push much at rest, you will have less farting.
Make sure you're not way over the sweetspot. Just because the valve doesn't open far enough for good velocity doesn't mean that it can't burp.
All of that is just additional info to the posts above. I think it all fits together.
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Last edited by Spider!; 01-29-2013 at 10:38 AM.