Any screw with a tension spring behind it is some sort of adjustable screw. My guess is that one is an idle mix screw and wouldn't have much, if any, effect beyond idle..
There's no downside to using gaskets, especially where the carb or intake manifold is involved. .. since someone was messing with that carb there's a chance they left it out. Any slight air/vacuum leak in this area can have a profound efffect on performance.
Get a few new spark plugs to play with .. and it sounds like that carb needs cleaning or float level adjustment if there is one available .. the float is not shutting off fuel for some reason or other. The result is a rich air:fuel mix that would tend to foul the spark plug. Low RPM uses a richer mixture than high speed, where a leaner mix is required, and high speed would suffer most..
Search this forum for info on the Dellorto carbs .. they seem to attract a lot of attention.