I've got a 79 F250 with the 300 straight 6. Just put a new carb on a few weeks ago and am still working on getting the idle speed set right. Ive got the high/cold idle set about right but when it comes to the low i am still having troubles. For some reason it's like there is 2 low idle speeds and it sometimes switches between the 2. So I will have it idle nice and smooth and then it will be too low and go between the 2 whenever it wants.
Is there an adjustment im missing or can you think of somthing I might be doing wrong?
I am new at this sort of thing so any help would be appreciated.

 

Well, I don't know exactly about the 300, but on my 302 the carb has a few different tweaks and it essentially has 3 different idle speeds and they are related to choke operation.

The first step is when you get in the vehicle and set the choke it sets it on a very high idle to aid in initial startup. After that, assuming you don't let it idle long enough for the choke to completely pull off, the first time you actuate the throttle it will fall to another idle position. This will be slower than the initial, but still faster than the fully warmed up idle. Eventually when the choke pulls off completely you'll land on the base idle position.

I am not sure but I assume the carb for the 300 would work similarly. It sounds like you accidentally set your "base" idle while it's on the intermediate position. Thus it works fine while it's cold but when the choke finally pulls off completely it's trying to idle too low. Best bet would be to make sure none of your linkages are binding then drive it for a while and make sure it is getting completely warmed up and the choke is fully open, THEN set your base idle and see if the problem persists. I'm betting it won't because I basically fought the same thing myself.

 

What your describing sounds about right except that it's too low when its cold and too high when its warm. As far as I know there are only 2 idle adjustment screws on my carb so im not sure which low idle im making the adjustment on.

 

Hmm... well it sounded good. My idles speeds and such were kinda flaky until I went through my adjustments. Turns out that I had the choke pull-off adjusted too far open. This meant that it would start immediately but it would idle way too high until it stepped down. I adjusted that with the main idle screw to get a good cold idle but then the hot idle was too low. I got that adjusted and now it really seems like the startup and intermediate steps are not really adjusted independently of the base idle, rather they are just steps above it in terms of % throttle opening.

If it's too low when it's cold it sounds like your choke pull-off is not opening far enough and you're having to compensate for that by increasing the main idle screw too high. Then when the choke gets warm and gets out of the way, your main idle is open too far and thus it's idling too fast.

Honestly that's just a shot in the dark though as I have never touched the 1bbl for the straight 6. I am assuming it has some kind of a choke pull-off that is similar to the 2bbl but I could be dead wrong.