actually i have an 86 GS450L that had almost that same problem, i used to just slip the clutch a little bit with no gas while idling to make the revs drop. but what i found out was that i had adjusted the idle speed while the bike was still warming up, so i just turned it down a small bit after the bike was warm. also on top of that i found that my carb clams werent very tight, so tighting them up seemed to help a small bit. good luck