Motorcycle Repair: Honda 550 carb question, honda 550 four, vent lines

Question
1974 honda 550 Four.  Carbs professionally rebuilt, new fuel, tank rust free, has extra inline fuel filters and stock petcock screen - all clean.  The bike runs good except when I open the throttle past about he first quarter inch, the bike sputters a little.  What part of carbs should I have checked?  The carbs are synchronized.

Answer
Marc,
The part of throttle operation you are describing takes place in mostly the slow main circuit and the needle is starting to lift out of the main jet. The sputter would be caused by not enough fuel or too much. Set the idle mixture screw at 1 ½ turns if not there already and warm up the bike then adjust till you get a good smooth idle. Try adjusting out the screw (more rich) a ¼ turn at a time and see if the sputter goes away. If it gets worse it might be too much fuel and try going leaner. If you can’t get it to adjust at all or makes no difference in idle turning the screw all the way in you have problems in the carb for the idle circuit. Make sure your carb to intake boots aren’t cracked and are in good pliable condition. Make sure vent lines are unobstructed and choke is shutting all the way off.  Float levels can change this also. Start with the mixture screw and let me know what it does.

Eric