The mixture screw (fuel screw) is right underneath at the carb flange to manifold end ......... OKO's , KF's , Keihins and genuine VM-Mikunis have an easy to get at AIR screw up on the side of the carb on the air filter side .......
Here's a link here which refers to carbs with fuel screws .......
ThumperFaq: MXA Fuel Screw
I've explained in detail how I tune idle air mixture in quite a few threads and my method works spot on very quickly ........
I always correct carb to manifold / manifold to port and gasket mismatch problems first ...... plus get the exhaust side set right as well ....... I actually go a step further and remove all sharp edges (detonation points) from the combustion chamber , valves etc and smooth and blend the ports ...
IF you have fitted that "26" mm Mikuni to a 70 with a smaller intake port with mismatches and it has a pipe less than 26 mm I/D ....... then good luck living in bog and stumble city when you open the throttle .......
To dial a carb in you have to first get it to idle steadily and smoothly ...... THEN with the needle set in the middle position ...... you do a full throttle thru top gear test and change main jets until the engine peak revs crisp and clean ....... then if it has a hesitation or bog off idle or when the throttle is opened up quickly you need to change pilot first then possibly mess around with the needle taper and slide cutaway until you get it working to suit you ....
My engines usually crank hard and clean after the first two steps with a #38 Keihin or the Mikuni equivalent pilot jets and a #100 to #105 Keihin size main jet ....