the idle screw you speak of, is it the idle mixture screw that someone has uncovered by drilling out the hole. or are you speaking of the throttle adjustment screw on the side of carb. people sometimes confuse the two. one controls idle rpm while the hidden one contols the mixture.
here's a couple of first steps to take on diagnosing your particular problem.
the first test to make would be take the tank off remove plug...... what color does it read. that is the best indication i know of. others here are better equiped than I to explain the differences in detail. in a nutshell it should be clean, light colored with no substantial build up. spark plugs are like motor oil, wax, and butt wipe

. they all work pretty much the same, but we all have are prefrences. you choose what works for you.
second have someone follow you at speed to report on emmissions.
next at 75 did it feel like that's all it had in it or did you just get to your limit. it should do mid 80's
i think what you will find you have are some minor tuneup type of issues which can easily be overcome.
with the help of people on the forums you will find you can do just about anything that comes up with this simple machine. this way you save a ton of cash by doing it yourself, and that translates to more money for [glow=red,2,300]FUEL [/glow].
keep us posted
welcome
sluggo