I have an 86 Z28 305ci, 4bbl. I have the idle screw backed off and it isn't even touching the stop. Even if I pull the air cleaner holder and force it, the idle won't go down past 750rpm's. And unless I hit the throttle fast when I stop, it will idle at about 1000rpm's. What can I do to fix it? Thanks.

 

It is quite often a large vacuum leak that causes a high idle speed, expecially on an old carbureted engine. My brother's 85 Z-28 had a MASSIVE vacuum leak on the hose that goes from the vacuum port behind the carb to the vacuum reservoir that feeds the HVAC stuff under the dash (to open and close the various doors/flappers under there). When fixed, idle speed dropped from 900-100 (with idle screw backed all the way out) to about 400 RPMs such that we had to open the throttle slightly via the idle speed screw to bring the idle speed back up into spec. There are miles of vacuum hoses on these old carbureted engines so there's plenty of places you can pick up a vacuum leak.

Another common reason for a high idle is that the choke isn't opening all the way. The choke engages a "high idle cam" on the side of the carb that increases the idle speed anytime the choke is on (even slightly). Look for a metal lever between the choke element and the pass side of the carb- a big beefy sucker with a sorta "triangular" shaped head on the end. Push DOWN on this arm until it stops. If your high idle cam or choke is stuck this will IMMEDIATELY drop your idle down to it's lowest setting, if it sin't there already. Then at least you know if you've got a choke-related high idle problem or not.

If you try this stuff out and you're still stuck, or you just need more info you can email me and I'll try to help further. Carbs are my strong suit and I've worked on a zillion QJets- computer controlled or not.