Idle mixture screw

[oddball]

My 85 truck failed smog today because of high HC and CO levels, the NOx level was low. It leads me to believe that its running too rich. Since the idle mixture screw had been tampered with I tightened it and then gave it 3.5 turns out. The FSM says not to tighten too much so my question is how tight is good enough? Also besides the obvious what else could cause it to run rich? The fuel level in the carb is a little high but within the maximum level.

[Duffil]

you really only need to snug it up...anything further can damage it. you can "engineer" a vacuum leak by pulling the hose off of the valve that controls the hot air intake valve,which leans it out a bit,  that did it for me, also, check out this thread

http://board.marlincrawler.com/index.php?topic=1119.0
hopefully you went the pretest route!

[oddball]

 I read that a couple of years ago but thanx,,,,,,,,,,,,Ill just try the mixture screw and hope its the problem

[MiniSimp]

Also don't forget to adjust your TPS, make sure your charcoal canister is clean, adjust your valves, then....adjust your idle.

[crawlerdan]

is it efi? if it is, you can lean the mixture by adjusting the mass airflow sensor, its easy and free.there is alot of sites on the web that will go through detailed how to