I am running a recently rebuilt Autolite 4100 carburetor and a Duraspark II ignition system in my 1985 302.

I warmed the engine up and set the idle and the dash pot at 750RPM. It idles great here, until I take it for a spin. When I come back, the idle is up to 900RPM. I give it a quick stab of the throttle, and it comes back to 750RPM. Another testdrive, and the same thing happens again. I pulled the air cleaner and it appears that the throttle isn't returning all the way to the dash pot setting. If I manually push the throttle arm down to the dashpot, it idles fine again at 750RPM.

I take it for a spin again, and the same thing happens. Frustrated, I turn it off for about 30 minutes. Go back to start it, and it stalls immediately. Try it again and the engine starts right up but now it is idling low at 500RPM (but touching the dash pot that was previously set at 750 RPM) and is about ready to stall. So I give it some gas and the idle comes back to 750RPM and stays there until I drive it or rev it up high and then it wants to idle at 900RPM again. I have to give the gas a quick stab again to get the throttle to return to 750RPM.

It was running perfect until yesterday, and the engine was rebuilt only a couple of weeks ago.

What in the heck is going on with this thing?

 

Sounds like linkage is binding somewhere. Either from pedal or on carb itself. Also mark position of dashpot to see if it's changing on you.

 

Thanks, gfw1985.
I checked and rec-checked, and the dashpot is not changing. I even removed it to eliminate that problem. I realized the dashpot wasn't the problem after I removed it and the idle wouldn't drop back down immediately after revving the motor up.

The distributor is connected to ported vacuum, so it wouldn't be advancing at idle at all. Just to be sure, I unplugged it and the problem is still there.

About the linkage binding. Where do I start? How do I check the linkage from the pedal to the carburetor? I already checked the return spring, and it has plenty of tension to push the throttle back to the dashpot. Is there anything else to check? The carburetor is an almost new unit from Pony Carburetors, so it shouldn't be the carburetor, but I guess anything is possible.

 

Yeah, I'd disconnect pedal from carb and check carb itself. Open and let it close slowly to see if it hangs up. Not all I've heard about Pony has been good lately. Seem to stand behind their product good though. At the least, that'll narrow it down a bit.

 

Thanks for all of the suggestions, gentlemen.

I resolved the sticking throttle issue and the choke is working perfectly.

However, I am still 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?

 

This may sound coming out from left field but is your rig equipped with AC?? ...... could be activating and engaging the compressor clutch and killing the idle.

Other possible issues include a temp sensitive throttle shaft that warps when heated; maladjusted butterflies (gets hung up in the throttle bores), or a sticking/maladjusted choke piston.

$0.02

 

No A/C.

how do I inspect the throttle shaft to see if it is warped or for maladjusted butterflies?

Why do you think the choke piston could be causing this problem, when the choke is opening up and closing normally?

 

You'll need to pull off the carb and inspect it. Some rebuilds require the throttle shaft to be re-bushed. You could have an intermittent vacuum leak. Wiggle the shafts to see if there is any play. There should be none.

With the butterflies, pull off the carb and check for any unusual wear in the throttle bores from their opening and closing. A shiny spot will tell you there is a defect.

It's opening and closing but is anything hanging up that is causing what I presume to be extra air being introduced when it is not supposed to? I trust rebuilds to a certain degree but there is always a chance that something was overlooked. Humans are humans.

How about this.... try running two carb springs - one on each side of the throttle action. I had to install a secondary spring on my 70 because the arm wouldn't return to its idle position without a lil help. I also ditched the funky bell crank's dual action, throttle linkage and now only have one arm from the gas pedal lever (mounted on the firewall) to the carb. I dunno what system yer running but take a look at the linkages. Maybe something is binding.

 

Just an update. I replaced the throttle cable, and the linkage isn't binding anymore.

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 just discovered something that I don't think is normal:

I replaced my throttle cable a few days ago and was under the hood checking to make sure I was getting full travel at the carburetor. The engine was cold and hadn't been driven in a few days. After opening the throttle all the way a couple of times, I noticed fuel was seeping at the spring for the throttle linkage. I dried it up and opened the throttle again, and fuel ended up on the throttle spring again. I tried to move the throttle shaft to see if it was loose, but it is tight. I checked around the entire carburetor and noticed that the power valve gasket looked shiny, like it was wet.

What would cause fuel to seep out at the spring for the throttle linkage? Is this what is causing the erratic idle I am experiencing?

 

Are you talking about the spring that's on the shaft? If so, it's probably seeping between the throttle shaft and throttle body. This is a common issue on older carburetors. The throttle linkage is probably holding the shaft in place enough to keep it from looking loose, but if you were to completely disassemble the carburetor, you'd probably be able to see some play.

It's not ideal, but many old carburetors have this issue. I suppose if it's bad enough, it could cause a vacuum leak that could lead to idle issues, but this is unlikely. The power valve gasket should not be wet.