I'm going to have to go ahead and guess that there's a pretty good chance this will not even work unless the perfect
spring is used.
In a Shocker, the air being used to fire the ball is "balanced". It does not influence the bolt to go forwards or backwards. If there was no solenoid air you could theoretically air up the gun and then freely move the bolt back and forth. When the gun is working normally, the solenoid air is transferred from in front of the bolt (pushing it backwards) to the rear of the bolt (pushing it forwards).
Here is an animation. The blue air is used to fire the ball. It is not influencing the bolt to move forwards or
backwards. The pink air is controlled by the solenoid. As you can see, it is either pushing backwards only
, or pushing forwards only
. During firing it first vents, and then energizes the opposite chamber.
When the air is holding it back, there is plenty of force even with a spring pressing forwards because no air is pressing forwards
, only the spring. The spring just helps get the bolt moving instantly
once the air holding it back is vented, even before the solenoid can switch and start pushing the bolt forward. It speeds up the reaction time.
In a Geo the bolt is not balanced. The bolt has the same amount of air pressure acting on the front and rear of the bolt, at the same time. The only thing holding the bolt back is a difference in area for the pressure to act on. With a larger area on the front of the sail, the same amount of air pressure is able to exert a greater force. The plus side to this, is that as soon as the solenoid starts venting, the bolt will start moving. You don't have to wait for the solenoid to send air into another chamber to move the bolt.
So, in the case of the Geo it is not pressure vs. nothing. It is pressure vs. pressure with a slight difference in area creating the rearward bias when aired up. If the spring is too strong it will offset this delicate balance and the bolt will stick forward and vent all your air. If it is too light it will have little to no effect.
If you are able to simply drop in a spring and the gun even works, the best case
scenario is that it will simply speed up the bolt's forward movement. The exact same thing the ST kit was intended to slow down. IMO, if you want a faster bolt, just put the stock can back in and shoot a louder gun with a bit more kick. You can lower your dwell and feel better because someone on the internet told you lower dwell = better.
For a spring to be effective in changing efficiency it would have to be implemented in a much more complex manner that actually has an affect on the valve resolution of the firing cycle. Getting the bolt to move forward more quickly is not really the issue here. The ST kit has shown that (some people actually get better
efficiency with a slower bolt, but no one gets worse
Feel free to try it, but any difference is most likely going to be a decrease in performance, or at best placebo effect