Hi, I am not 100% sure, and I no the gurus will be able to help better, but the two possible options are (if available on your model) the stop screw on the carb holder plate/stay, this can be adjusted to increase idle, or when balancing the carbs, if your has the lock nut and adjustable nut to set the balancing, this too can be set to raise the needle higher, thus increasing the idle? but wait for the top members to confirm.
http://www.cmsnl.com/honda-cb750k0-four-general-export_model14352/partslist/E26.html#results
No 4 and No 40
good luck
John
Could be several things that affect idle speed and not just carburators.
Is it running on all 4 cylinders ie all 4 exhausts hot.
Is timing ok and points and condensers ok.
Is it drawing air, spray some wd40 or something similar around manifold rubbers then around each carb in turn. it will increase revs if drawing air.
Is the smoke ok out exhausts or is one or more carbs running rich or weak also check spark plugs all should be a nice light brown colour.
The simple thing is that you are not adjusting the idle screw on the carb or carbs mine has one screw/hand wheel that adjusts all four. if you are turning the mixture all over the place then that's wrong any more than a half turn then something is wrong.
try these and see how you get on.
