Typically, stock bars measure 7/8". Your left side grip should measure 7/8" inner diameter, and the right side grip should measure 1". This is because the throttle mechanism needs to slip to do its job, and the plastic portion is slightly bigger than the handlebars.
Measure away on both ends of the old bars and your new ones, and let us know what you find.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.