TOPIC:

Does the adjustable screw on the stock mikuni carbs ajustthe fuel and air or is it only the air it adjust? 
getting fowled plugs

The screws at the front of each carb adjust the idle speed fuel air mixture. In is leaner, out is richer.  These are nominally set to 3.5 full turns out from fully seated and then adjusted from there. The O-rings on each of the adjustment screws can perish (especially with ethanol based fuels) and allow extra air in.

On later model carbs the main jet needles have 5 circlip grooves that allow for some adjustment. Early models only had a single circlip groove and could not be adjusted

All correct; with  a side-note.  Fuel will never come in contact with the pilot-screw O-rings.  Do not try to solve your fouled plug issue by tinkering with the main needle height until the idle fuel/air is adjusted correctly. A Gunsons Color Tune sorts this out if you can't  adjust the Pilot Needles by ear (smoothness of idle) and by "reading" the plugs. 
I have no idea as to what-all has been done to your carbs but another thing you can do is remove the guts from the Enriching Valves - replacing the diaphragms with a solid gasket. 

wrote:

All correct; with  a side-note.  Fuel will never come in contact with the pilot-screw O-rings.  D 

My mistake. Maybe it was the O-rings supplied with my aftermarket carb rebuild kits that were too small.  My idle mixture screws were loose to turn in their ports.  Tightened up a treat after replacing the O-rings.  I had wondered if they were drawing in air past the O-rings that were too small/perished

I don't think there's any question that the Pilot-needle O-rings purpose is to seal for air infiltration. Given, however, the very FINE threads; the air infiltration with old O-rings would be negligible.
Naturally, a viable O-ring  is preferred.  For what it's worth:  I use a little silicone grease on the O-rings and threads. It preserves the threads and additionally,  provides a further level  of sealing..
 

As this seems to be a general A3 carb thread,  how in the hell do you adjust the mixture  screws when you can't get a screwdriver in there, being blocked by the choke rod mounts?