SRS stock CR125 shifter packages are supplied with carbs jetted slightly rich for density altitudes about 1000ft and ambient temperatures around 75 degrees. The main jet will usually need to be reduced about 5 points to attain the best lap times. Variances in temperature up to about 15 degrees will require only a main jet change. However, drastic variances in temperature and or density altitude from the 1000ft/75 degree range will require slide needle position and air screw/pilot jet changes to return to optimum performance.

Here is an actual example. On a 75 degree/1000 foot day one racer jetted his Keihin carb 170 55 DEG-3 for best lap times. When racing in the Texas heat with temperatures of 100+ degrees and a density altitude of 3200 feet, he ended up with a 162 52 DEG-2 setting. The main jet changes were for wide open throttle performance. The slide needle and pilot change were to keep linear, controllable power when the throttle was rolled on exiting turns.