Do you have a tachometer to properly lower said idle speed?
Idle speed is roughly 1750 to 1850rpm. Check YOUR engine manual for proper speed.
Why/what are you trying to idle down for? These are NOT car engines. Maybe on a full pressure lube engine? Most are splash lubed. Briggs says to run at max revs all the time for MAX durability, longevity, proper COOLING and lubrication. MAX revs is roughly "around" 3600rpm.