1984 yamaha virago 750cc- where is the air fuel mixture screw?

Did you try to increase the idling adjustment screw?
Check the spark plugs for its condition and thn reset the air-mixture screw.
Standard procedure for setting air-mixture screw...tight it all the way in, taking care to not over tighten or the tip of the screw may be damaged, thn turn outwards 2 & half to 3 turns, start your bike's engine ,run it to normal temp. and the reset the screw + - alittle either way and you should notice the difference in the idling and all.
Its better too, to also check the valve clearance !
Hope this helps!

http://www.stadiumyamaha.com/pages/parts/viewbybrand/2/Yamaha.aspx

look up your bike here and i think it is number 13 on the diagram. which looks like its right by where the throttle cable connects