Low velocity on Custom Pro

I am having problems with low velocity on a Tippmann 98 Custom Pro E. It has an expansion chamber and regular hopper on it. It did have a cyclone, but I took it off and closed off the port on the side. Can't get velocity above 220 fps. Tried different tanks, same result. Do you think i need a stronger bolt spring? It does have ACT as well.

Thanks for any help.

Edited by k4dv - 29 January 2008 at 10:15am

Do you have any other after-market stuff on it?  (Stocks, barrels, etc.)

Try a stiffer drive spring. This often helps. It hasn't helped me, but most other cases of this are a weak drive spring. Also, check that the detent isn't in backwards.

When you say the velocity screw is "all the way out" what exactly do you mean?

It very well could be the drive spring as the I-man said, but there are other possibilities as well.

The velocity screw on Tippmann markers works by interfering with the free flow of propellant from the valve to the chamber.  This means that the approximately the same amount of gas is used for every shot and ball speed is controlled by how fast it is released.  Essentially, the farther in you turn the screw, the more in the way it is and the slower the air is released.  The nature of this adjuster means that there is a point (approximately four complete turns out from all the way in) where screwing it any farther will no longer increase velocity.  In fact, depending upon other variables (like how tight the screw fits in the valve) turning it out farther can cause air loss and decreased velocity.

I suggest turning the screw all the way in, then backing it out three complete turns and working from there with a chrono.

I am still curious as to what type of barrel you are using.  Longer barrels tend to require more air as do barrels with tighter bores.  If you were using a long barrel with a tight bore, the combination of these factors could also cause the problem you describe.  In that case your best solution would be an after-market spring or rear velocity adjuster.  Backspin barrels tend to require additional air as well.  Finally I have seen barrels that didn't fit well (on older "broken in" markers) work loose and lose pressure around the end of the barrel at the receiver.