I am experiencing three different idle situations.

The engine idle was initially set to 750 RPM (500 RPM in gear)when the engine was driven for about 45 minutes and was fully warmed up.

The choke works fine. When the engine is just warming up and at full operating temperature, the idle is right on at 750 RPM. In gear, the idle is at 500 RPM. This is right where I want it to be.

If I drive the truck for a while, say 15 miles or so, the idle wants to drop way down to below 500 RPM at idle and in gear (AOD), then the idle will waver down-up, down-up, down, down, and finally it stalls. Then, when I try to re-start, the engine won't catch because the idle is too low. I have to give it gas when I start to have enough engine idle, but then when I let my foot off the gas, the engine will "catch" at about 500 - 650 RPM again.

Turn the truck off and wait 20 minutes and the idle is back to normal at 750 RPM. If I get under the hood and rev the engine, the idle will sometimes return back to 750, and other times it wants to drop down to 500RPM.

One thing I have noticed, is that when I turn my vent controls on, they work fine but the idle drops down just slightly. Then when I turn the vent back off, the idle will go back up slightly. I never noticed this before. Is that normal?

 

The vents shouldn't effect the idle unless you have a leak somewhere.

In a separate issue, I would check the float levels when the idle issue starts. Could be a little too high causing to to slightly flood itself as the carb heats up.

 

I think I may have found something...
I put my hand over the carburetor as the engine was running and it did not stall. So I guess that means there is a vacuum leak somewhere. I went messing around, and discovered a "hissing" sound coming from around the PCV valve. I pulled the PCV valve off the valve cover and the idle did not change much at all. I would have thought this would be a MAJOR vacuum leak? I shook the PCV valve and it rattles, and there is a rather strong suction on my finger. But, I can turn it around in the grommet and the "hissing" sound changes. And sometimes when I remove the PCV valve, I hear a LOUD "sucking" sound coming from the valve itself, and at other times there is no sound at all. In both cases, the PCV valve still has strong suction on my finger.

Does that mean anything?

 

It may be sucking too much. They are cheap, replace it and see what happens.

 

Sounds like PCV valve isn't sealing to valve cover. Grommet is probably old and brittle. Thats the suction side of PCV system so you will feel it pulling vacuum. Vents: If you have a vacuum gun, you can apply vacuum to hose leading to vents and see if it holds vacuum or bleeds off.

 

Does that carb have the throttle stop solenoid on it? The one that (can't think of the right name at the moment) holds the throttle at the set idle speed until the key is turned off then lets the throttle close... Maybe its got a bad connection if it still has one on it...

 

The wavering idle almost sounds like binding/stuck linkage, or a dashpot or idle stop solenoid acting goofy (mentioned above).

 

Just an update. I did find that the PCV valve was getting pushed out of the grommet because the hose was too long, so I cut it back and solved the minor vacuum leak that was there.

However, the idle problem remains. If I put my hand over the air horn, nothing noticeable changes with the idle. I don't hear any "hissing" noises that would be indicative of a vacuum leak. All bolts and screws are tight. I do not have an EGR valve. There is no hesitation or stumble at all when I take off.

I set the idle at exactly 750 RPMS, and it stayed there most of the day. Then, on the way home, the idle started "wavering" again at idle and in gear. Then I found that the engine decided it wanted to idle lower than what I originally set it at. I revved the motor up, and it cleared up a little. Then it idled great for another 5 minutes. Then suddenly, for no apparent reason, the idle took a hit and immediately dropped to around 500, almost as if the A/C compressor turned on, like a load was suddenly placed on the engine. But my A/C isn't hooked up.

 

I have seen cheap PCV valves change their position to the point the flow varied from what it should be at idle to almost none or huge vacuum leak. Make sure your secondaries aren't staying slightly open sometimes, gently push the lever on the choke side closed and see if your problem duplicates. When I used to overhaul these carbs, if I removed the throttle plates, there was a procedure to reset their position and the stop screw. These were also bad about failing to close fully if the rod from the secondary diaphram was bent, or the diaphram wasn't installed correctly.

 

Where is this lever you are talking about located?

 

Sounds like the float level is just a tad too high and after idling for a but, it starts to flood itself a little. Not 100% sure on this.
Do you have an extra return spring on the linkage? Might be a good idea to add one if not. It may not always be returning completely.

 

On the choke side of the carburetor there are two levers, the one directly inboard of the choke has a screw in it and a small plastic pin that serves as a keeper for the screw and a tang to push the secondaries closed. Behind and inboard of that is a sort of three pointed lever that has one of the points bent in slightly. This is the secondary throttle shaft end. This is the one you want to make sure is going all the way against the stop. With the engine not running, hold the primary throttle wide open, push the lever coming out of the back of the carburetor forward until the secondary throttles are wide open. Let it go, it should return to the point the tang on the end of the shaft hits the stop screw in the carburetor base, you should be able to hear a faint click when it does. If it doesn't go all the way closed then either the diaphram is too tight or the rod from the lever to the shaft is bent too straight.