Throttle not dropping fully down to idle 4180C - going crazy

[85stanggt]

Hey guys

I have a stock 4180C with the secondary metering block mod. The car runs great and idles smooth.

What's driving me crazy is the idle. Something is holding it up and keeping it from dropping down. I'd say it's been an issue for a while (on and off over the course of the last couple years). It seems to only happen when the engine is fully hot. I know there is a system to increase the idle to help keep the fan speed up to cool the car, but this is higher than that. Not to mention, it's been in the 50s or 60s when this has been happening, so that system shouldn't even be in effect. Normally, I cure the condition by lubricating the throttle linkage and hitting the throttle spring and stuff with a little PB blaster.

Unfortunately, now most recently the car has been idling even higher, and despite doing my usual routine and even blasting everything clean with carb cleaner, it still holds itself up.

The normal idle is set to 700rpm after the choke is pulled off. When the car is started, it runs on fast idle, and the choke completely pulls off as it should down to the 700rpm idle.

After driving the car a little and getting the engine up to full temp, I notice the idle is holding at about 1000rpm after pulling up to a light. I can blip the throttle and get it down to about 850-900rpm. If I lightly press the accelerator and raise the rpms up past 1000 and let it go, it will hold again at 1000rpm until I blip the throttle.

I can physically push the throttle pin down while the car is running and get the idle down to 750rpm. I can see that there is like 1mm or less of travel in the A/C solenoid that the throttle can't seem to compress when the car is running. While off, it seems to compress fine. ???

Additionally, if I physically push the secondaries closed while the car is running, the idle drops completely down to the expected 700rpm. This only seems to happen when the car is running. Starting the car from sitting (even hot), the idle is normal until the car is driven.

I hope this all makes sense. It seems very touchy. And it's going to drive me insane soon if I can't figure this out. Is this a physical throttle / binding issue or is this a vacuum issue??

Thanks

[qikgts]

The throttle return spring got weak on my car so it developed an issue similar to yours. I simply added a return spring to the linkage and now it fully closes every time. It did make the pedal effort go up some.

[85stanggt]

That crossed my mind. Which spring are you talking about? The one on the carb baseplate or the long one that is part of the throttle cable? I thought maybe the spring on the baseplate was weak, but didn't think of the throttle cable. Does the cable itself ever have to be lubricated?

What do you think about the secondaries? Why wouldn't they be closing all the way?

[dynodon64]

There was a recall from Ford for this reason back in the day. The problem you already found when you mentioned manually pushing secondary closed. The problem stems from a vacuum signal going to the secondaries when it shouldn't be. On the drivers side of the carb, there is a small linkage rod that goes from the throttle on the primary shaft to the secondary shaft. If you look close at that link, you will notice some slop in it the that if you try to turn the secondary shaft it will move a little. If you stretch out that link a little you can remove that slop. This will help hold the secondary shaft closed. That's the purpose of that link. The secondaries won't open until the primaries are past a point.
Give that a try and let us know if it works.

[Walking-Tall]

The spring inside the secondary diaphragm is what holds the (preferably non-binding) secondary throttle shaft and plates in their proper idle position. There isn't any vacuum signal up in the throttle bores for secondary actuation during idle or low speed, that happens much later to actuate the secondary at WOT, with the required velocity going through the throttle bores and past the small pitot tube/channel/s up in throttle bore/s that connect with the secondary diaphragm. The purpose of the above-mentioned link is to begin the closing of the secondary throttles when letting up from wide open throttle. It's dangerous potential binding and throttle sticking territory, and can create primary idle rpm variances, to use that side link to be positively closing and holding secondaries closed. The secondaries will not open until past a point of load and velocity happening through the primaries, and later and fully with signal from the primary pitot tube and a small secondary bore orifice in older (BETTER) Holleys, at WOT.

I'd check to make sure the secondary throttle shaft isn't binding, being held open, and if so, why. Hold the primary open and move and check the secondary for free movement, and for return to complete closure all by itself, which is the job of the secondary diaphragm spring. Check the link between the primary and secondary for for a twist or binding or something and opening the secondaries some when it's not supposed to.

You mention both primary and secondary maybe not closing at idle. The "throttle kicker", or "idle air control", or "A/C solenoid" (or whichever it is yours has, if adjustable), may very well need checked/adjusted to where it needs to be.

[qikgts]

That crossed my mind. Which spring are you talking about? The one on the carb baseplate or the long one that is part of the throttle cable? I thought maybe the spring on the baseplate was weak, but didn't think of the throttle cable. Does the cable itself ever have to be lubricated?

What do you think about the secondaries? Why wouldn't they be closing all the way?

The cable spring lost some tension and it was only the front throttle plate not completely closing.

I missed the part about the secondaries and never had an issue there so I have no experience with that but it seems others have offered info that seems pretty sound.

[scarecrow]

My god. My 670 street avenger is doing the same crap! I thought it was my choke being crazy but nah, that's not it. I don't have a silly A/C throttle kick up thing either.

[85stanggt]

There was a recall from Ford for this reason back in the day. The problem you already found when you mentioned manually pushing secondary closed. The problem stems from a vacuum signal going to the secondaries when it shouldn't be. On the drivers side of the carb, there is a small linkage rod that goes from the throttle on the primary shaft to the secondary shaft. If you look close at that link, you will notice some slop in it the that if you try to turn the secondary shaft it will move a little. If you stretch out that link a little you can remove that slop. This will help hold the secondary shaft closed. That's the purpose of that link. The secondaries won't open until the primaries are past a point.
Give that a try and let us know if it works.

That's the first I've heard of Ford recall about that. I had adjusted that metal rod ever so slightly to take out slack. I put it back to the way it used to be because I had a feeling it may have been delaying the opening of the secondaries, as I could kind of feel them open at 3k+ rpm. It actually feels better leaving a little slop in there, which I think must have been intentional by Holley? I was always under the impression that the secondary spring was intended to close it, but there's nothing else I could think other than binding that would prevent it.

The spring inside the secondary diaphragm is what holds the (preferably non-binding) secondary throttle shaft and plates in their proper idle position. There isn't any vacuum signal up in the throttle bores for secondary actuation during idle or low speed, that happens much later to actuate the secondary at WOT, with the required velocity going through the throttle bores and past the small channel/s up in throttle bore/s that connect with the secondary diaphragm. The purpose of the above-mentioned link is to begin the closing of the secondary throttles when letting up from wide open throttle. It's dangerous potential binding and throttle sticking territory, and can create primary idle rpm variances, to use that side link to be positively closing and holding secondaries closed. The secondaries will not open until past a point of load and velocity happening through the primaries, and later and fully with signal in a small secondary bore orifice in older (BETTER) Holleys, at WOT.

I'd check to make sure the secondary throttle shaft isn't binding, being held open, and if so, why. Hold the primary open and move and check the secondary for free movement, and for return to complete closure all by itself, which is the job of the secondary diaphragm spring. Check the link between the primary and secondary for for a twist or binding or something and opening the secondaries some when it's not supposed to.

You mention both primary and secondary maybe not closing at idle. The "throttle kicker", or "idle air control", or "A/C solenoid" (or whichever it is yours has, if adjustable), may very well need checked/adjusted to where it needs to be.

So, how do I know if it's binding? I've held the primaries completely open, opened the secondaries, and let it go. When the car is off, it slowly closes completely as the diaphragm release. Or so it seems...we're talking about such a tiny amount of movement at the end of travel, it's hard to see. But if there was a binding problem, I feel like it would be obvious with them getting stuck noticeably open part way through travel.

So I'm figuring it has full, unimpeded travel from it closing up without help. However, with the car on, I can open the secondaries within that "slop" of the safety bar as mentioned above and let it go, and it does what I describe. It closes down because the rpm drops back down, but I can push it a *little* more and hear the idle fall more. Now, keep in mind, we're talking like 50-100 rpm. The primary is way worse. So if it's not closing all the way with the engine on, why would that be? Within acceptable for the small rpm change?

The cable spring lost some tension and it was only the front throttle plate not completely closing.

I missed the part about the secondaries and never had an issue there so I have no experience with that but it seems others have offered info that seems pretty sound.

I don't think the primary vs secondary issue is interrelated. The majority of the issue is from the primaries. If I could fix that, I'd be thrilled. Can the spring itself be completely replaced? I wonder if I could add a stop to the end of the spring to decrease the relaxed length, but I don't know if that would compromise total pedal travel length...I'll have to look. Maybe the cable needs to be lubricated?

[Walking-Tall]

That's the first I've heard of Ford recall about that. I had adjusted that metal rod ever so slightly to take out slack. I put it back to the way it used to be because I had a feeling it may have been delaying the opening of the secondaries, as I could kind of feel them open at 3k+ rpm. It actually feels better leaving a little slop in there, which I think must have been intentional by Holley? I was always under the impression that the secondary spring was intended to close it, but there's nothing else I could think other than binding that would prevent it.



So, how do I know if it's binding? I've held the primaries completely open, opened the secondaries, and let it go. When the car is off, it slowly closes completely as the diaphragm release. Or so it seems...we're talking about such a tiny amount of movement at the end of travel, it's hard to see. But if there was a binding problem, I feel like it would be obvious with them getting stuck noticeably open part way through travel.

So I'm figuring it has full, unimpeded travel from it closing up without help. However, with the car on, I can open the secondaries within that "slop" of the safety bar as mentioned above and let it go, and it does what I describe. It closes down because the rpm drops back down, but I can push it a *little* more and hear the idle fall more. Now, keep in mind, we're talking like 50-100 rpm. The primary is way worse. So if it's not closing all the way with the engine on, why would that be? Within acceptable for the small rpm change?



I don't think the primary vs secondary issue is interrelated. The majority of the issue is from the primaries. If I could fix that, I'd be thrilled. Can the spring itself be completely replaced? I wonder if I could add a stop to the end of the spring to decrease the relaxed length, but I don't know if that would compromise total pedal travel length...I'll have to look. Maybe the cable needs to be lubricated?

Yes, it's intentional. The mechanical link initiates and mostly physically closes the secondaries when you let off from kickin' 'er to the kitchen, lol, and then the diaphragm spring takes over for full closure to idle position.

Yes, you'd see binding, just like you've described. It's a slow closure due to either a check ball or a small restriction in the secondary diaphragm housing, but it's a closure, and the secondary spring's job, as well as regulating opening manners, is to hold the secondaries in their idle position. A side note is that throttle shafts can get a slight twist in them. The secondary idle position set screw is at the underside of the passenger side of the throttle base, working with the throttle bracket with tabs for idle position setting as well as WOT stop, and any forcefully (over-) closing with the link on the driver's side can put a slight twist in the throttle shaft, creating then a situation of one secondary throttle plate closing more than the other, which could account for the minor idle rpm drop when you hold them closed on the driver's side that little bit more like you describe. Or, depending how hard you're pushing, you could be twisting the throttle shaft a bit and closing the driver's side throttle plate that wee bit more...

You're right, they're not interrelated, unless that driver's side link arm has been bent to be unnecessarily shoving the secondaries closed, which in turn, if the unnecessary is overdone, can be holding the primaries open some off of it's idle screw. You could add an additional primary throttle return spring, or shorten (stiffen) your primary return spring if you have one, or add a shorter/stiffer one for better closure at idle. It might make for a stiffer pedal but it's good insurance. Unless of course this is an issue with the primaries getting told what to do when the car is running: "The "throttle kicker", or "idle air control", or "A/C solenoid" (or whichever it is yours has, if adjustable), may very well need checked/adjusted to where it needs to be."

[qikgts]

I don't think the primary vs secondary issue is interrelated. The majority of the issue is from the primaries. If I could fix that, I'd be thrilled. Can the spring itself be completely replaced? I wonder if I could add a stop to the end of the spring to decrease the relaxed length, but I don't know if that would compromise total pedal travel length...I'll have to look. Maybe the cable needs to be lubricated?

I don't know if it's worth putting too much effort and time into modifying it when a replacement is available.

https://www.npdlink.com/store/produc...8614-2457.html

The return springs I added came from the Help section at McParts so the cheap fix worked with basically no effort and cost.

[JACook]

I dealt with this problem a while ago with my '85 hatch, and it bugged me for longer than I care to admit.
I went through the baseplate, making sure all the throttle blades were exactly where they needed to be
(good exercise, btw), and that there was nothing binding. I made sure the tang on the secondary throttle
shaft, that sits on the secondary idle screw, was perfectly perpendicular to the throttle shaft centerline
(another good exercise). I even swapped in a new dashpot. Solved a lot of problems, but not the one I
was trying to solve.

One thing I noticed was if I would drag the idle speed down with the clutch, the idle would stay there,
which is when I started thinking it was just a weak return spring. I ended up solving the problem using
a slice of 5/16" fuel hose, placed behind the throttle return spring seat, at the end that attaches to the
bracket. No problems since.

[85stanggt]

Thanks everyone for the help. I'll attempt a fix of this after the holiday.

I dealt with this problem a while ago with my '85 hatch, and it bugged me for longer than I care to admit.
I went through the baseplate, making sure all the throttle blades were exactly where they needed to be
(good exercise, btw), and that there was nothing binding. I made sure the tang on the secondary throttle
shaft, that sits on the secondary idle screw, was perfectly perpendicular to the throttle shaft centerline
(another good exercise). I even swapped in a new dashpot. Solved a lot of problems, but not the one I
was trying to solve.

One thing I noticed was if I would drag the idle speed down with the clutch, the idle would stay there,
which is when I started thinking it was just a weak return spring. I ended up solving the problem using
a slice of 5/16" fuel hose, placed behind the throttle return spring seat, at the end that attaches to the
bracket. No problems since.

Thank you for the picture. This is exactly what I had in mind when I mentioned shortening the spring travel above.

I will take the carb off soon when I replace the intake gaskets, and double check the things you mentioned. When I rebuilt the carb, I think I remember adjusting the primaries to expose just a square of the idle transition slot when closed. I'll take a look at the secondaries.


To all: Are there any adjustments I should be looking at while the carb is off that would improve hot starts? As it stands now, starting the engine hot will cause it to stall unless I work the throttle a little. Then it'll idle at 500 rpm until, I guess, the carb cleans out and stabilizes. Don't know if the idle adjustments are interrelated to this issue. Carb is flawless otherwise. I'm pretty sure this idle return issue is not related and just a result of the spring being aged.

[82gt50]

Your signature says "1985 Mustang GT Convertible Stock and original @ 209k, except for dynomax ultraflos" so that carb has a lot of miles. When you remove it carefully check the throttle shaft bushings for play or you can spray a little carb cleaner where the shafts enter the plate while idling and see if it changes (vacuum leak) if the shaft bushings wear out they can bind and not close consistently. Also as always check possible vacuum leaks from secondary sources, for example engine runs fine then it warms to operating temperature and a coolant temp vacuum switching valve opens routing vacuum to a device that may be leaking causing idle problems.

As far as hot re-start problems the EGR plate just under the carb is really cooking things, the factory put a thin aluminum heat shield between the two but it only does so much. A 4-hole 1/2 inch plastic insulator just under the carb would help if you have the hood clearance. Also be very careful when tightening the carb and go light in sequence reassembly taking care not tweak the fragile base plate and bind the throttle shafts.

Edit: intended deleting this image but cant figure out how...

[82gt50]

I don't know if it's worth putting too much effort and time into modifying it when a replacement is available.

https://www.npdlink.com/store/produc...8614-2457.html

The return springs I added came from the Help section at McParts so the cheap fix worked with basically no effort and cost.

Agreed, beyond the spring loosing tension on the cable assembly the stranded cable itself can fray inside the housing and start to drag preventing smooth operation. This is a safety concern so replace the cable assembly if there's a problem.

[85stanggt]

Tackled this today. Driving around, idle was holding at 1000 rpm. 100% the cause of the primary side, because I could push the throttle down with my finger and have it drop back to normal.

Did a few things:
1) Stretched the spring out manually. I stretched segments of the spring at a time (like a few coils at a time) to try to reactivate the spring, since I couldn't easily get the spring off. It definitely helped, but not enough.
2) I sprayed a little PB Blaster to get things sliding more easily and remove binding
3) I put a nylon tie at the end of the spring as a spacer, much like what JACook did with the fuel line.

So far so good, for the few minutes I tested it, but I'll drive the car tomorrow and that'll be the real test.

[qikgts]

There ya go... I'm going to take the return springs off mine and try this.

Great job! Thanks for sharing!

[85stanggt]

I can confirm after driving it for 4 days that it idles perfectly now. I actually had to adjust the idle after this...up a little bit...and now the hot starting seems a bit better. That may just be a fluke that isn't showing itself until it is hotter outside, I don't know.

[gr79]

Similar problem:
Found out why the carb throttle was suddenly sticking. Solved. Was able to undo, repeat issue, undo.
Holley 2v, stock manifold, modded manifold adapter spacer. 5/16-18 flange bolts and nuts, carb to adapter.
Everything was fine until carb was removed yesterday for successful spacer adapter gasket mod.
Test drove fine, but throttle was sticking when trying to accel past 1/2 position of primary throttle plate.
Quick blip of pedal unstuck it. Did not stick like that before.

Was late, so slept on it.
Did not know why. Could find any one real reason searching online or in Holley Service Guide #2.
Pedal always seemed to feel a tiny tiny bit harder to push since new carb was installed in May.
Had to be something i did. But did not change anything major, non-standard, or do any experimenting.

Today
A thorough visual check of carb linkage and area turned up nothing abnormal. Floor mat was ok.
Removed pedal linkage off the carb. No change. Accell cable= eliminated as source of issue.
Pushed carb linkage to sticking position.
Loosened carb mount bolts. Throttle snapped back to normal idle position. Ah ha. Relief. But why?

Found the Holley carb mount holes allowed a tiny bit of rotational adjustment with the fasteners i am using.
Enough so binding could be duplicated turning carb cw or ccw all the way. Like a 32nd of an inch or something.
With nuts still retaining carb, rotated carb until linkage did not bind. Centered it, snugged it down.
Test drive ok. Pedal feels much better finally like normal smooth and easy.

While the carb was off, took the opportunity to measure the transfer slots/throttle plates vs throttle screw adjustments.
Got out the magnifying glass, narrow pocket machinist's scale. Sunny outside, good lighting.
Both primary curb idle screw and secondary stop screw moved the throttle plates about 1/64 for each 1/4 turn.
Marked one of the flats on the primary curb idle screw in red as an index.
With about .031 or 1/32" of the primary transfer slot showing, screw was 1.5 turns out from just touching carb base.
Read spec is about .040 max of primary transfer slot showing. Any more and idle adj is said to be iffy.
Secondary screw was not as precise to measure, as the linkage stop tang can be bent to adjust how it sits on the screw.
The secondary throttle linkage stop tang can be bent off perpendicular on purpose or not.
I reversed the secondary set screw prior so as to be able to adjust secondary throttle plate opening from the top.
Does not want to be turned in flush in its hole anymore. Custom bent a tiny screwdriver for adjusting the screw.

Idle is normal, no bogs, flats. Smooth transition from idle to full throttle and everywhere in between.
Nice steady high idle vac and stable idle rpm. Has to be close for this engine. Set by the book and went from there.
Had to back down the primary idle rpm screw 1/2 turn to lower the idle from 1300 to 1100. Idle mix not touched (yet).
Can drop the idle lower with the primary's curb idle screw. OK with that rather than secondary plate adj. It is what it is.
Leaving it for now as the weather and seasonal pump fuel blend is changing.