Hello,

been trying to remedy a “hanging” idle situation. I have the choke “cap” adjusted, fast idle screw slightly lower than the “V” so it’s around 1200 fast idle. Idle air mix screws have been adjusted to max vacuum (20-21hg) with curb idle set at operating temp.

Truck idles great at around 750-800. If I go for a drive and come to a stop or press the throttle down and hold it for a second or two and let off. The idle will hang at around 1200-1300. If I tap the throttle it’ll drop back down to 750-800. I checked to make sure the idle air screw and cam are not in contact and they are not under this scenario.

any thoughts? Something I’m missing here? Choke cap not fully letting off even when warm?

1973 F100, 360, 2100 carb.

thanks, Morgan

 

In my experience at least, the most common reason for that would be a sticking fast idle mechanism.
My usual quick fix is a quick spray with carburetor cleaner in the area behind the choke housing, then a quick spray of light lubricant like WD-40.
You’re not really supposed to lubricate that part I don’t believe, but after they get a certain age, they start hanging up sometimes.
Gravity is what lowers them in some carburetors, and the part itself is very lightweight. So sometimes, that little bit of stiction is enough to hold it above.

Normally, this only happens once per cycle though, so not common for it to hang up after just pushing the throttle.
But it can’t hurt to try a little lubricant anyway.

The main idle speed, and idle air mixture screws, don’t have any bearing on this fast idle mechanism. Only the fast idle screw and its associated cam steps.

 

What I've run into for sticking throttle idle like this is worn throttle shaft bores. It allows the blades to hit the side and get stuck, but sometimes they can snap shut and be fine. Usually a quick tap remedies the issue. Spraying cleaner around the throttle shaft can reveal this issue, but it may not eliminate it. The shafts can often get bent as well.

 

If you resort to an added return spring, put it in as near straight pull opposite the throttle rod for less wear on the shaft bores..

 

Thanks for the tips guys!

I sprayed some carb cleaner around both sides of the throttle blade shaft. Then sprayed some lube as well. Still have the issue.

it’s not a intermittent issue, it’s almost guaranteed every time the rpms get above 1200-1300. And idles down after I tap the throttle.

I want to relay to you guys that this issue came up over the last few days after adjusting the choke cap, and fast idle screw. I do not know if that is what is causing this but like I said in the initial post, the fast idle screw is not contacting the cam when this is happening. Choke flap is fully open(operating temp).

Could this have anything to do with the choke pull off tab being broken? I ordered a new one and am waiting for it in the mail.

Morgan

here is the truck…

 

I'd think a good chance, but without actually seeing it,

 

You didn't adjust the timing or base idle? I'm still thinking throttle shaft/blade issues as I've ran into this several times, including monkeying with the choke linkage ad nauseum until I figured it out.

Bad part is, you usually can only duplicate it while running, the engine vacuum contributes to the issue. Off the engine, it's hard to tell there is an issue.

 

I have it set to 10 base timing. Vacuum advance on manifold.

im beginning to think it may be the “dash pot” holding the idle open. I am new to these 2100s.

I adjusted the dash pot away from the linkage and it hangs lower now.

does anyone know if with a manual and no A/C that the dashpot is even necessary?

Morgan

 

One thing. Are you setting it to that 10° with the vacuum advance connected? If so, you realize that as soon as you open the throttle, the vacuum starts going down.
That lowers your timing even more. A little more throttle lowers it substantially to the point where you’re probably at 0°, and getting into the negative territory.
This would result in lack of acceleration, stumbling, and in extreme cases, even killing the engine momentarily.

That’s why I was asking before, about having to adjust your idle speed down after connecting the vacuum advance.
If you set it to, for example, 6° before top dead center, then connect the full vacuum to the canister, your idle speed would then (usually) go up because you’re probably at, whatever that high advance was you mentioned.
Then you’d have to reset the idle speed to bring it down to a normal speed with that substantially higher ignition advance.

This is the circumstance that often gives many of us fits when we try to use full manifold vacuum.
At least with ported vacuum, you set your timing to a known level, then, as soon as you start to push on the throttle, your ignition timing actually advances to a certain extent. Helping with acceleration
This is often more controllable.

I’ve read articles and seen a lot of YouTube videos about why full manifold vacuum is so much better. But I don’t believe I remember ever seeing anyone address the idle speed issue.

 

Sorry, my mentioning the stumble as something you had mentioned, was, I think a memory from another thread here.

 

10 degrees with vacuum advance unhooked.

 

So I totally removed the dashpot to eliminate it as holding the throttle open. No better. I tightened up the return spring, still no difference.

This evening I watched the driver side of the throttle blade shaft closely as I worked the throttle. It seems bent somewhat, or at least the throttle hookup seems to be off. If I physically push the throttle back it’ll hit the curb idle screw and idle down. I imagine the carb is original to the truck, is it about time it’s worn out?

I sprayed carb cleaner on the shaft and let it dry, then some lube. I’m going to let it sit over night in timeout.

is this carb junk at this point?

Do you guys recommend a reman 2100 or jumping over to a Holley 2300?