I own a 1988 Accord DX, 90,000 miles, Carb, 49 state emission spec. Am experiencing difficulty reducing the idle speed to spec. Idles at 1500 to 1700 rpm. Have cleaned distributor (cent. advance weights operate freely) and replaced vacumn advance unit. Replaced carb, vacumn hoses all checked and tight, general tune upcompleted, timing checked,etc. Turning idle speed **** on carb has no effect on idle speed. Any advice on troubleshooting, likely culprits, would be appreciated. Thanks.