Blow-back Balancing

As detailed on the Two-tube Blow-Back Basics page, these semi-auto paintball guns function by releasing a dual burst of gas from the valve to both propel the ball and re-cock the action of the gun.  Problems arise when there is insufficient gas to perform both duties, or the amount of gas being used is not properly balanced between both duties. These problems usually manifest themselves in one of two ways; The inability to attain a usable velocity, or the inability of the gun to re-cock. The conventional method of overcoming the problem is to change springs until the valve is releasing enough gas to overcome a deficit in any one direction. However, conventional methods are not always the most efficient, and sometimes do not even work, especially when dealing with low pressure operations.
The common problems described above are the types I encountered when working on the low pressure conversion of one of my Spyders. The conversion included the use of a large Shock Tech volume chamber, and a TASO turbo valve kit. The turbo valve kit came with a new valve, in which the top half had been basically removed to improve gas flow, and a valve pin which was turned down to allow more gas flow. The parts were placed in a Bob Long Millennium Spyder replacement body, which is the full size body style, as opposed to the more commonly seen compact Spyder bodies. My goal was to get the gun operating in the neighborhood of 350 to 400 psi. At an indicated 400 psi, the gun was shooting 345 fps, which of course is way too hot. By manipulating the springs or lowering the operating pressure, I could get the velocity down, but then the gun would refuse to re-cock. No mater what kind of combination I tried, if I lowered the velocity, the gun would stop re-cocking once velocity was brought below 320 fps or so.
What was the problem here? Fortunately, from my experience with the Line SI Promaster, I realized that the problem was an improper balance in how much gas was being used to propel the ball and how much was being used to re-cock the gun. Afterall, I obviously had adequate pressure and volume if the gun functioned well at 345 fps. The problem was in how that pressure and volume was being distributed. While most people would combat the problem by increasing the pressure, then using spring changes to limit the volume for a usable velocity, that method would not only defeat my goal of operating at 350 to 400 psi, but would also be less efficient than using what resources I already had available.

Far left, a stock Spyder valve pin, middle, an aftermarket turbo valve kit pin, and right, a home modified valve pin.

Since I obviously had too much gas being used to propel the ball, the solution to the problem was to re-direct some of that gas for re-cocking. The most cheaply reversible and easiest drop-in method of re-directing the gas is to modify the shape of the valve pin. The shape of the valve pin determines how much of the gas entering the valve is directed upwards to propel the ball, and how much will be left to escape out the front to re-cock the action. While the stock valve pin would not allow enough volume of gas to travel upwards for low pressure operation, the turbo valve kit pin actually allowed too much, due to it’s radical cut.  Thinking somewhere in the middle might work better, I took a Dremel tool and modified a another stock pin, cutting the shape down, but nowhere near as much as the Turbo Valve kit pin. I wanted more volume up top than the stock pin could provide, but not as much as the Turbo pin allowed.

Using the same set-up that had resulted in 345 fps, the addition of the modified valve pin dropped the velocity by over 20 fps while continuing to allow the gun to re-cock. I had successfully re-directed some of the gas volume and pressure from propelling the ball to re-cocking the gun – in other words, I had balanced the system (or at least come closer to it). Once the system was more balanced, a simple main spring change dropped the velocity down to something useful. Main spring changes have less of an impact on re-cocking than most people realize. While it’s true that a weaker spring will cause the release of less gas, keep in mind that it actually takes less gas to blow-back a weaker spring.

With the way the gun is set up now, it is easy manipulate the velocity using only pressure adjustments on the regulator. The gun will now re-cock while shooting anywhere from 240 fps and up (the need to function at a low operating velocity was necessitated by night play in scenario games). A velocity of 290 fps requires an indicated 375 psi.

Not all improperly balanced systems need more volume and pressure for re-cock. Some, such as Neild Bingham’s PT Enforcer needed exactly the opposite. While the gun re-cocked with no trouble, velocity maxxed out at 180 to 190 fps. Check out the PT Enforcer Velocity Improvements page for his unique solution to the problem.