runs great but feels like half throttle

[doughboy19]

I have a 1971 mercury 500 50hp. Every part has been tested, new rings ( 125psi on all cylinders), will run in gear finally ( after link and scync from this forum), idles smooth. When i hammer down it seems to run half throttle no cutting out. I looked at the butterfly vales on both carbs and they open all the way when at the house but in the water not quit getting there. I set the timing with the plugs out of the motor only #1 in at timing specs. I still have to idle the motor up to take off a little more then timing was set. could the max advance have to be set a little higher or is it a cable problem. Both cables seem to move freely but with a little slack. Do i need a new cable or is the max advance to low it is set at 35 degrees.