Not coming all the way down to set idle

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TrailBeast

TrailBeast

AKA Mopars4us on Youtube
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I thought I took care of this possibility when I got the carb.
Whenever I get a new or different carb I always loosen the butterfly screws and make sure they center in the carb throat all the way. (seen to many holding one open)

After getting everything all set and driving, the engine doesn't quite idle down to what is actually set at unless I lightly tap the throttle with the tow of my shoe.
I like as light a return spring as I can get that fully returns the throttle, so I thought "well I guess I'll have to use a tighter spring than I had on it".
Nope didn't do it, and actually didn't seem to make any difference at all.

The carb isn't old and/or worn really at all, but I'm still assuming it has to do with the shaft being slightly off center from the spring putting sideways pressure on it when it comes down from higher RPM and hanging a butterfly slightly, and then when I tap the pedal it centers and idles down all the way.
Nothing drastic, just about 50-75 RPM.

I have a linkage disc instead of an arm so that all the connections of the throttle, TP cable and return spring can easily be changed or adjusted for geometry so I think I may have a solution.
Being that I use a disc I should be able to use two lighter springs One from each direction to equalize the off center pull from have one spring.
I'm going to try it and see, so think it'll work?

Yea or nay? :D

linkage.jpg
 
I assume you have the port at the bottom front (that shows in your pix) capped off? As well as other unused base ports?

If the prior carb idled down OK, then I'd suspect that the secondary stop screw is open more than it should be. Or there are holes drilled in the primary plates.

Otherwise, I would also be looking at the PCV flow at idle vacuum. Pull it with the hose attached and put your finger over the opening on the bottom of the PCV and see if the engine idle drops. What vacuum level and waht PCV are you using?
 
BTW, I may not be understanding.... do you set the idle, and then it does not hold there later? Is the idle being initially set with the engine just partly warmed up?

Are the throttle shafts tight in their bores?
 
nm9stheham said:
BTW, I may not be understanding.... do you set the idle, and then it does not hold there later? Is the idle being initially set with the engine just partly warmed up?

Are the throttle shafts tight in their bores?
Click to expand...

I think you might not be, sorry if it wasn't clear and I'll explain better.
Fully warmed up with a totally stock unmodified Edelbrock 1406 with electric choke.
Very slight play in the shafts (maybe 5-6 thou) and a couple of years on the carb.
All ports either used or blocked (that one port in the pic is the blocked off non ported vacuum)
No intake leaks or odd stuff with the PCV.

Just simply that the throttle hangs just slightly above whatever I set the idle RPMs at until I lightly tap the pedal, and then it comes right down where it is supposed to be. (800 RPM) or 850 when it hangs.
I was thinking that equalizing the spring pull on the linkage might take care of that, and that even though the carb doesn't seem worn at all it still may be the off center pull on the linkage causing it.
It's really a very small deal, but it bugs me when things like idle speed vary from one stop to the next. :D
 
Well, I ain't no rocket scientist, but I think you are on the right thought process. In looking at that wheel thingamajigger, I would 'think' moving the spring up toward the top of the wheel may help. :eek:
 
Bad Sport said:
Well, I ain't no rocket scientist, but I think you are on the right thought process. In looking at that wheel thingamajigger, I would 'think' moving the spring up toward the top of the wheel may help. :eek:
Click to expand...

Did that already and it didn't make a bit of difference. :D
Like I said earlier, at first I thought I just needed and stronger spring and that didn't do it either.
Just made the pedal stiffer, which I don't like as it just wears things like the shafts even quicker.
I'm pretty sure equalizing the side pull on the throttle shaft from one spring pulling it one direction is causing it.
Even if I do have only .005 wear on the throttle shaft bushing/surface it's still going to hang a plate .005, or 50 RPM right? :D
 
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mderoy340 said:
Check the linkage at the pedal and lube the throttle cable with thin oil.
Click to expand...

It's already loose and smooth.
The clip on the end of the cable end at the carb even falls slightly down at closed throttle.
I just went out and moved the return spring to the bottom of the disc as a first experiment, so now the spring pulls back on the bottom of that disc instead of forward at the top.
We'll see what that changed and go from there.

Using that disk instead of a one arm linkage gives a lot more options and makes changes quick and easy. :D
That's why I designed and built it that way.
 
I only asked about the bore tightness as that is another possible source of air leakage. Shoot some silicon lube in the shaft bores and see if it clears up.
 
I'll just throw a couple of things out there, that I'm sure you have already checked. These are for future readers using the search button.

1) the T-port sync: If the butterflies end up too far open, the Vacuum Advance port may be activated pulling in some timing, which will always increase the idle speed. The toe-tap may be just enough to drop the signal out.
2)the t-port sync: if the timing is too far advanced, then the throttle plates may park very closed to fully closed. In this position, the idle will be very sensitive to very small changes in throttle parking. It will usually be accompanied by a hesitation at throttle tip-in.
3) the T-port sync; If the butterflies are open a lot too far , the transfers may not want to quit. This usually manifests as the rpm taking an abnormally long time to return to the preset idle. But it usually does return to idle after some 2 or 3 seconds. Occasionally tho, it just hangs. The proof of this situation is to kill the ignition for a second and reconnect at just below the preset idle speed. If it then remains at the preset, that is the proof.
4) the weights in the dizzy can hang up when very light springs are used.Extra advance means extra idlerpm.
5) PCV. if the PCV does not park accurately, it can show up at idle.The PCV at this point is a controlled vacuum leak. If it doesn't park the same everytime, the idle speed will vary.This, however does not usually respond to a toe-tap.
6) linkages. Sometimes the links on the passenger side, make trouble. Especially the one to the secondary butterflies.There is usually a clearance spec for this link that is supposed to be reset every time the primary cracking screw is adjusted.
 
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nm9stheham said:
I only asked about the bore tightness as that is another possible source of air leakage. Shoot some silicon lube in the shaft bores and see if it clears up.
Click to expand...

Oh, the throttle blades rod bores in the carb body?
Nice and smooth with very little bore play.
Read the reply to AJ and it'll get even more obvious what it is/ was:D


Thanks AJ and I'll reply within your quote so it so it doesn't get all scattered.
And as an FYI the initial timing is 24 with an 11 mechanical in by 2,500 and it starts,runs and responds perfectly. (I normally run 35-36 total at this elevation.)
It is up around 50-52 when full ported manifold vacuum comes in under light throttle cruise.



AJ/FormS said:
I'll just throw a couple of things out there, that I'm sure you have already checked. These are for future readers using the search button.

1) the T-port sync: If the butterflies end up too far open, the Vacuum Advance port may be activated pulling in some timing, which will always increase the idle speed. The toe-tap may be just enough to drop the signal out.
Nah, don't think this is it because it's like 50-75 rpm and if it did pull some in this engine with it's timing settings would idle slower I think from too much.
2)the t-port sync: if the timing is too far advanced, then the throttle plates may park very closed to fully closed. In this position, the idle will be very sensitive to very small changes in throttle parking. It will usually be accompanied by a hesitation at throttle tip-in.
No throttle tip in or hesitation at all, and zero signs or sounds of too advanced. (I checked AFR and plugs both before satisfied)
3) the T-port sync; If the butterflies are open a lot too far , the transfers may not want to quit. This usually manifests as the rpm taking an abnormally long time to return to the preset idle. But it usually does return to idle after some 2 or 3 seconds. Occasionally tho, it just hangs. The proof of this situation is to kill the ignition for a second and reconnect at just below the preset idle speed. If it then remains at the preset, that is the proof.
The butterflies come against their bores pretty much at the same exact time the Idle speed screw contacts the stop, and the engines returns to idle immediately upon letting off.
Also when it hangs up it's got about 1/1000000 of and air gap betwwen the screw tip and the stop. (I checked) :D
4) the weights in the dizzy can hang up when very light springs are used.Extra advance means extra idlerpm.
Checked that also and not the case (Idle base timing is solid as a rock up to about 1500 before it starts to move.)
5) PCV. if the PCV does not park accurately, it can show up at idle.The PCV at this point is a controlled vacuum leak. If it doesn't park the same everytime, the idle speed will vary.This, however does not usually respond to a toe-tap. Exactly
6) linkages. Sometimes the links on the passenger side, make trouble. Especially the one to the secondary butterflies.There is usually a clearance spec for this link that is supposed to be reset every time the primary cracking screw is adjusted.
Click to expand...
Didn't check the gap between the linkages but I did verify nothing on that side is causing it.
Everything is free and clear with a small gap at the secondaries actuator lever.

I think this is what caused it.
Since the butterflies contact the bore walls at exactly the same time the idle arm on the linkage contacts the idle screw, it lets the plates contact the wall and that slight wear is letting them get jambed there by the pressure of the throttle spring pulling forward on the entire linkage assembly (rod, plates and all).
The blip of the throttle lets the plates center exactly and that last bit of throttle opening leaves and it idles where it's set for.
Keep in mind here that the amounts of play in the rod and the amount that the idle hangs most people would never notice I don't think. (Especially the linkage rod part)
Since I decided this was what was causing it I swapped the single for two 50% lighter springs (one at the forward top of the disc pulling forward, and one at the rear bottom of the disc pulling back) should solve that , and it did.
Now there is almost no forward or backwards pressure on the butterflies rod at all since the two springs are equalizing each other and only working together to close the throttle.
Even better for the carb this way so rod bore wear shouldn't be a problem for quite some time. :D

Nice list of possible reasons though.
Thank you.

P.S. What is it with you and T ports?
One would think they are important or something. :D

Just messin witcha. LOL

 
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I can almost guarantee you that the 24 initial is the root of all evil. but you have made it work so it's hard to argue with positive results.

But I can't help myself;
If, I say IF; somebody was to reduce your initial to something like 18, and then bring the mechanical in to start at 1000, then as soon as the engine comes off idle, the curve would start to come in and Ima thinking by around 1800 you would have the same timing as before the reduction. This would allow you to open up the butterflies, to maintain the idle speed that you lost with the reduced idle advance.This would of course put the t-blades a little further up the transfers and so would require a small reduction in fuel from the idle circuit.
The result of all this is;
1) a little less pump-shot required to overcome the dead transfers, and
2) a tougher bad-boy idle, and
3) the ability to bring in the Vcan a little earlier,a little faster,and possibly a little more of it. And a little more of it, makes less-jumpy slow-speed operation, and a smoother cruise, in second/third, and more mpgs on the highway, in top gear, and
4) if you have that fancy A-500 with a 2400plusTC, none of this matters. :)
5) much
 
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AJ/FormS said:
I can almost guarantee you that the 24 initial is the root of all evil. but you have made it work so it's hard to argue with positive results.

But I can't help myself;
If, I say IF somebody was to reduce your initial to something like 18, and then bring the mechanical in to start at 1000, then as soon as the engine comes off idle, the curve would start to come in and Ima thinking by around 1800 you would have the same timing as before the reduction. This would allow you to open up the butterflies, to maintain the idle speed that you lost with the reduced idle advance.This would of course put the t-blades a little further up the transfers and so would require a small reduction in fuel from the idle circuit.
The result of all this is;
1) a little less pump-shot required to overcome the dead transfers, and
2) a tougher bad-boy idle, and
3) the ability to bring in the Vcan a little earlier,a little faster,and possibly a little more of it. And a little more of it, makes less-jumpy slow-speed operation, and a smoother cruise, in second/third, and more mpgs on the highway, in top gear, and
4) if you have that fancy A-500 with a 2400plusTC, none of this matters.
Click to expand...

I knew you were going to say that.
Then it would make the thing run on after shutdown but the amounts you are talking about it probably wouldn't. :D
 
And I knew you were gonna say that! lol
With your cam size, run-on should not be an issue. If it rears it's ugly head, I would rather cure that then run the transfers dry.
But you have made it work so it's hard to argue with positive results.
 
OK, that explains that. But you a situation where such fine work on the springs should not be necessary. If the blades are so close as to touch or almost touch the bores to get down to proper idle speed, then something is not right. Sorry, but the throttle plates should be slightly open like .015", not right up against the bores.

I still suspect that there is some air getting in that you need to shut off, so that the primary blades will be open more at the right idle speed. Look down in the secondary bores and see how closed off those are; if they are propped open some, then the secondary stop screw should be turned down on a Holley. I never had a 1406 so don't know how the secondary stop is managed (besides the lockout linkage from the primary).

I have also had a loose primary to secondary lockout linkage (the one that keeps the secondary closed 'til the primaries are open to X degrees; it this what you are calling the actuator lever?). If that is loose, then the secondaries will pop open a bit regardless of any secondary stop setting and let more air in and raise the idle speed; you won't see this unless you look when the car is idling. I think you said this linkage was a bit loose? If you tighten it up to about a .001" or .002" tolerance, the secondaries will be sure to be closed. Again, I am talking Holley talk but the 1406 has to have similar link that has to keep the secondaries closed down properly at idle.

And pinch that PCV hose or cover the PCV's inlet at idle and see how much that effects the idle.
 
nm9stheham said:
OK, that explains that. But you a situation where such fine work on the springs should not be necessary. If the blades are so close as to touch or almost touch the bores to get down to proper idle speed, then something is not right. Sorry, but the throttle plates should be slightly open like .015", not right up against the bores.

I still suspect that there is some air getting in that you need to shut off, so that the primary blades will be open more at the right idle speed. Look down in the secondary bores and see how closed off those are; if they are propped open some, then the secondary stop screw should be turned down on a Holley. I never had a 1406 so don't know how the secondary stop is managed (besides the lockout linkage from the primary).

I have also had a loose primary to secondary lockout linkage (the one that keeps the secondary closed 'til the primaries are open to X degrees; it this what you are calling the actuator lever?). If that is loose, then the secondaries will pop open a bit regardless of any secondary stop setting and let more air in and raise the idle speed; you won't see this unless you look when the car is idling. I think you said this linkage was a bit loose? If you tighten it up to about a .001" or .002" tolerance, the secondaries will be sure to be closed. Again, I am talking Holley talk but the 1406 has to have similar link that has to keep the secondaries closed down properly at idle.

And pinch that PCV hose or cover the PCV's inlet at idle and see how much that effects the idle.
Click to expand...

I did check all that already, and I'm positive there is no extra air coming in from anywhere, and I have the PCV really limited so it makes very little difference when plugged.
The eddies have a mechanical lever that pushes another lever to open the secondaries (and there is space between them) and nothing being held open.
I think AJ pretty much hit it on the head but it runs and idles down so nice now I don't want to go back and redo everything just to get back to this same point (only with possibly less timing and more wear on the butterflies rod and bore.)
 
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