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 test drive, 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?

 

You might want to check the fast idle setting on the choke. Sometimes that can prevent the throttle from returning to idle.

 

Lariat 85,

Since you mentioned that the carburetor was recently rebuilt, this is most likely not the case, but a worm throttle shaft will cause eratic idling [vacuum leak].

 

I don't get what you mean?
The fast idle setting is set to 1500RPM. The choke is opening all the way, and the butterfly plate is straight up. This carburetor worked prefect until yesterday.

 

Is it possible that the distributor has a problem [i.e. sticking advance]? That could explain the undesirable increased idle speed, but I am not sure how that would explain the described drop to 500 RPM idle.

Has anything [i.e. mounting bolts, air filter] been tightened such that the carburetor might have shifted or slightly warped?

What about the throttle cable or gas pedal? Could they be binding?

 

The dashpot wasn't the problem because 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.

I already checked the throttle return spring, and it has plenty of tension to return the throttle back to the dashpot. Is there anything else to check? How do I check the gas pedal for binding?

 

If the throttle return spring has sufficient tension and the throttle is fully returning, then the gas pedal is most likely okay.

As you pointed out, if the vacuum advance is connected to ported vacuum, then it will not be advancing at idle if everything is working as it is designed. My thought was that the vacuum cannister and its associated linkage, might be binding, such that when the vacuum is removed [at idle], if the "stuck" unit does not return to its proper position, the timing might still be slightly advanced and kick the idle speed up.

If the linkage was stuck when you set the idle @ 750 RPM and then freed up, it could explain the drop to 500 RPM, I suppose. Let's see what others think.

 

I see what you are saying, 1986F150six, and that makes total sense. I'll try another test run with the vacuum advance unplugged and see what happens.

But would a sticking vacuum advance physically stop the throttle arm from completely returning to normal idle (and the dashpot) on its own?

 

No, it would not prevent the throttle arm from returning. That would have to do with something sticking or binding with regards to throttle cable, throttle shaft or some associated linkage on the carburetor, itself.

Grab the throttle plate shaft with your fingers and see if it can be wiggled back and forth. If it does, it indicates wear and the possibility of binding and or vacuum leakage at that point.

Also, it just occured to me that you have a four barrel [I, on the other hand have the MIGHTY six with its one barrel]. I have heard of instances where the secondaries stick open [even slightly], thereby causing havoc at idle???

 

I believe there are three stages of idle, Fast Idle (1600RPM), 2nd (1000-900 RPM) & Curb Idle (700-500). This is just my observation on 1985 F250 460 4180C Carb. By the time the engine is warm up, the curb idle will take over until the temperature goes down again.

Another possibilities to look at is the deceleration vacuum diapram, a sticking shaft from the diapram could give you same effect. triple check the cam linkage on the carburetor that is consistently going back at the same spot, only the RPM change. If that is the case, like suspect you might have a vacuum leak messing with your advance timing.<O</O

 

About the choke Lariat, I don't know if your 4100 has an automatic choke or manual
choke, but I have in the past encountered problems sometimes when the adjustment
screw for the fast idle may contact the first step on the fast idle cam depending on how
the choke is set. This can occur when an automatic choke is exposed to cooler weather
and might still have some spring tension when the choke appears fully open.

 

As mentioned above, a Holley has multiple fast idle "steps." Even if the engine is not warm, you can push down the end of the plastic fast idle cam and it will take the carb off of the fast idle steps and allow the thottle to hit the curb idle screw. Then the next time you touch the throttle it will jump back to the desired setting (whatever the choke tells it). Your Autolite is likely the same, and I bet the fast idle cam is holding the throttle open.

 

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?

 

Fuel Mixture may be set too lean or too rich. (I'm thinking lean)

Also check for a possible vacuum leak somewhere. (This can also lean out the fuel mixture)

These can cause an eratic idle that will change as you described in the last post.

As for the throttle linkage, lube the linkage ball stud. Using carb cleaner washes all the lubricants off the linkages and makes them sticky. Owners manual states to use a multi-purpose lubricant. Also the throttle cable can benefit from a good squirt from inside the cab and at the carb. Check the plastic ends of the throttle cable for breaking or cracking plastic.

 

Just an update. Fuel mixture is set correctly. I replaced the throttle cable, and the linkage isn't binding anymore. The choke is working as it should.

However, the idle problem remains. If I put my hand over the carburetor 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 is completely removed.