Well,....If your servo is centering and the geometery of the control hardware is good,......turn on the radio and move the rudder in both directions,..check to see which cable is more relaxed with the rudder deflected. You may find out one cable is slightly longer than the other. Also make sure the control harware at the rudder is the same on both sides of the rudder,....from the hinge line back and the height is the same. The transition time and centering of the sevo should not change normally,..however if your cables are not adusted close to the same lenght the transition lenght is not even and one side pulls less or more than the other (slop) and the word "slop" dosen't mean it has to be signficiant.

Don't be affraid to tighten the cables,..the servo will let you know when it's unhappy and you have gone to far,...it will constantly buzz or humm,...back the cables off equal turns on both sides,...move the rudder left and right with the radio on and check center again,..repeat this as many times as it takes to make center in both rudder directions.

I spend more time tweeking my rudder cables than anything else in the beginning during set up,...be patient and good luck with it.

Also,...make sure the servo plate is not moving under a load ( twisting, etc),...have someone put a light amout of pressure on the servo and see if the plate is bowing or twisting. I have seen this happen on several entry level type ARF pattern planes and non-pattern type planes and it will drive you nuts trying to figure out a centering issue in flight.

Regards,
Bill Holsten