Stock 77 xs400 cable doesn't seem long enough

[brok13]

I have just finished rebuilding this xs400 , I have not changed the bars or cable yet when I hook the cable up onto the perch at the carb's it cracks the throttle too much and races up when started . I have re-routed the cable multiple times and it's all the way adjusted to the longest length . Any suggestions as to what could be happening ? It worked before.