Two things come to mind:
1. There is a tiny brass orifice in the 657A/B carbs, in the air horn area, that is the air jet to the emulsifier tube. When the carbs sit dry for a long time, this passage gets a white corrosion (powder) inside and even a 25% blockage can create very, very rich midrange problems. Remove the carbs, look at the air horn end, and you will see them: remove the bowls and the emulsifiers (that's also called the "mainjet holder) and run a flexible wire from the air horn end into the emulsifier: you'll see it enter and bump into the needle jet when it gets there. Do this several times, to push the crud through, then spray and clean well in those tiny passages.
2. Keyster neeles and/or needle jets in the slides. The only way to tell is to take them out and get the number off of the needle, report back her to someone who might know if it is Keyster. Their needles for this '72 bike are hopelessly wrong.
A final thought: see which clip slot the needles are in, just in case they are stock Honda parts. On the K2, they should be in the 2nd slot from bottom. If rich midrange is an issue, they can be moved to the center or even 4th slot, although the 4th slot will start to make the engine pop when the throttle is pulled open at 1/4 throttle or more.
The earliest K2 had 110 mains in 657B carbs. The later K2 had 105 mains in the same carbs. My K2 had the 110, and constant plug fouling problems at about 1000 miles per set. The 105 solved that: here in Colorado (6000 feet) I run a 100 and regular gas in town, midrange in mountain touring, and premium if I go to lower altitudes. If I ever go on a sea-level excursion again, I'll go to 105 before I leave town, and adjust octane according to altitude on the trip: lower altitude = more octane.
