High Idle speed after fixing accel pump on Holley carb. Looking for ideas while I think this through.

I thought I was so smart. My car normally starts with one pump of the gas. When it didn't and I noticed an off idle stumble while in park (I didn't drive it) I knew it was the accel pump. I look down the carb and it was confirmed. Replaced the accel pump and now I have a high idle speed. I checked fuel levels, made sure the throttle wasn't stuck open, like a linkage issue. The choke doesn't work but I verified it isn't binding either. Adjusting the idle speed screw (not the mixture screws) had no impact in either directions. It was hot and I was tired of sweating so I shut it down and went home to think about it.

Plan of action:
Verify I didn't create a vacuum leak. The most likely culprit would be the pcv. I know I hooked it up to the carb but perhaps it came off at the valve cover, mine is hidden in the passenger rear by the engine bay struts. . I will also check that I didn't mess up a vacuum plug. The number of vacuum connections I use on the carb is very limited. Just the pcv and the outboard carbs, which isn't manifold or ported.
Next i will take a peak down the carb throats to confirm the butterflies are all closed.
Then I will spray around the carbs with some water but this isn't a small leak, it's idling as high or higher than a fast idle setting on a working choke. So I doubt this will 'find a leak'
Finally I will test the timing. I use manifold vacuum for the distributor off of a manifold port. I will unplug it just to see. And I did have to remove a spark plug wire at the cap to get the front carb off, I didn't feel the distributor move but anything is possible.

For a moment I thought perhaps it is the power valve but that causes a rich condition, it wouldn't cause fast idle. Same if any of the fuel bowls were overflowing. The only reasons I can think of that will cause a fast idle is 'more air', from a leak or the throttle open or from an advanced ignition. As I mentioned, it didn't get any faster (or slower) speed from turning the idle speed adjustment. Yeah, I am leaning towards a big vacuum leak. Which would be nice.

Any input from the gang?

Check that your fast idle cam on the choke side isn't hung up.

will e said:

I thought I was so smart. My car normally starts with one pump of the gas. When it didn't and I noticed an off idle stumble while in park (I didn't drive it) I knew it was the accel pump. I look down the carb and it was confirmed. Replaced the accel pump and now I have a high idle speed. I checked fuel levels, made sure the throttle wasn't stuck open, like a linkage issue. The choke doesn't work but I verified it isn't binding either. Adjusting the idle speed screw (not the mixture screws) had no impact in either directions. It was hot and I was tired of sweating so I shut it down and went home to think about it.

Plan of action:
Verify I didn't create a vacuum leak. The most likely culprit would be the pcv. I know I hooked it up to the carb but perhaps it came off at the valve cover, mine is hidden in the passenger rear by the engine bay struts. . I will also check that I didn't mess up a vacuum plug. The number of vacuum connections I use on the carb is very limited. Just the pcv and the outboard carbs, which isn't manifold or ported.
Next i will take a peak down the carb throats to confirm the butterflies are all closed.
Then I will spray around the carbs with some water but this isn't a small leak, it's idling as high or higher than a fast idle setting on a working choke. So I doubt this will 'find a leak'
Finally I will test the timing. I use manifold vacuum for the distributor off of a manifold port. I will unplug it just to see. And I did have to remove a spark plug wire at the cap to get the front carb off, I didn't feel the distributor move but anything is possible.

For a moment I thought perhaps it is the power valve but that causes a rich condition, it wouldn't cause fast idle. Same if any of the fuel bowls were overflowing. The only reasons I can think of that will cause a fast idle is 'more air', from a leak or the throttle open or from an advanced ignition. As I mentioned, it didn't get any faster (or slower) speed from turning the idle speed adjustment. Yeah, I am leaning towards a big vacuum leak. Which would be nice.

Any input from the gang?

Click to expand...

Put a vacuum gage in place to confirm that you do have a vacuum leak.

If the idle screw does little to nothing you more than likely have a leak that is UNDER the throttle blades, air has to be getting to the engine somehow in order to raise rpm. Maybe a bad seal to the manifold, make sure the bolts are only hand tight, and verify you didn't crush AND have the gasket in the orientation, ask me how I know. An easy way to check for a leak is to place your hand over the primaries to cut air flow to the carb normally, if the idle doesn't die out or even change it is a vacuum leak, but like I said if the throttle blades don't do anything the leak is under them.

In terms of readjusting the holley here are some tips in my few rebuilds.

Power valve would be a separate issue. If you have touched the curb idle screw at all I would verify that the transfer slot is squared off. Incase you don't know what that is or what I am talking about it is the long rectangular slot that is in the primary bores and under/being hidden by the throttle blade, unfortunately the carburator has to come off in order to flip it upside down and dial it in but it is worth it. You just want to screw the curb idle screw until the butterflies only show a tiny perfect square of the transfer slot when shut. If you turn the curb idle screw too much it'll open that transfer slot too much or not enough which can cause a bog. Once you have it squared it is best if you never mess with the curb idle screw specifically.

In terms of holleys they are all pretty similar just want to be careful cause they are fragile. To dial in your floats you want to make sure the carb is full of fuel, turn on electric pump or crank the engine a bit, open the sight plugs (if they aren't the glass kind), proper level is when gas spills out if you give the car a shake or two, if you have a 4 barrel there are two sight plugs total. If they aren't at a good level you need a flathead and a wrench and fiddle with the bolt on top of the float bowls. Be gentle cause you will wreck the gasket, ask me how I know. Also if you have an electric fuel pump you need to make sure you are not above around 7 psi of fuel pressure I believe. Also be careful with the fuel inlet they strip out easy, ask me how I know.

Fuel mixture screws can safely be turned all the way in to seat gently then I think its 3 full turns out as a good base line if they are messed up at all, turn in either direction to get highest/most stable rpm.

This was just a quick write up with some tips to find the issue, if you need something more specific I can elaborate more.

Did you remove the carb to fix it?

If so make sure gasket isn’t hanging the blades open or you didn’t warp the base not tightening the bolts in a crisscross pattern

I had the exact same issue a couple months ago after I removed the carb for the purpose of changing the carb spacer. Long story short, I discovered a VERY SMALL tear in the carb gasket that was creating a vacuum leak. Replaced the gasket and problem solved. It doesn't take much to create a vacuum leak and throw the idle out of wack that's for sure.

I have a video on a 2100 2v carburetor where I show how to adjust the high speed idle screw for when the chole is (parially) on. You may need to adjust it, or check to make sure it is able to drop freely when the chick is warmed up and upen. I am not sure what carb you are running, but on a lot of carbs it is pretty challenging to find and adjust the high speed idle screw.

I hope the video helps:

https://www.youtube.com/watch?v=mwLwwUtI1cE

Still working on it.
More clues:
When I remove the hose PCV from the carb base there is little to no change in the idle speed, even if I use my finger to plug and unplug the port
When I remove the brake booster hose from the manifold port there is a noticeable change.
When I was checking the choke cam I moved the throttle open/closed to see if it was 'hanging', it isn't HOWEVER a small amount of fuel escaped from the throttle shaft. It was getting wet from the accel pump shooting fuel into the carb and appears to have leaked out a bit.
It diesels when i turn it off. This indicates the additional air is getting mixed with fuel so it's probably being drawn in from the carb itself (or the base seal).

It's a 50 year old carb, I don't know all of the history on it, but I am leaning towards the shaft leaking. It's always idled a bit high and on occasion it has dieseled. Perhaps pulling it off and farting around with it made it even worse.

I am going to pull it to verify the butterflies are closed as it sits and to recheck the gasket.

I might swap it to another car just to see how it idles on something different, this will confirm if it is the carb.

At it again this morning.
Plugged every vacuum port. (PCV, transmission ,Brake, distributor, vacuum gauge, everything including the outboard carb lines)
Turned idle speed screw down all of the way down (counter clockwise). Way past where the plate stopped moving.
Confirmed outboard carbs are not held open by linkage
Verified timing is at 12 using timing light
Cold it doesn't diesel but warm it still does. I added a shutoff valve to the fuel supply to 'kill' the engine.
Still get 1050 RPM in park, warm, no vacuum advance on dist with idle speed turned all they way down.
I checked the idle mixture screws, they were a little over 1 1/2. Turning them in to 1 had no effect except perhaps the idle wasn't 'as smooth'
I COVERED the center carb (with the Air cleaner off) using both hands and it had almost no impact at all. (Thanks for the ideas Zach).
I sprayed carb cleaner all over the place and it had little to no effect.

Next steps:
Pull the carbs.
Put plugs in the outboard intake holes and try idle again with just the center carb. IF I can get the idle to drop we know it's something with the outboard carbs.
If I can't get the idle to drop I will test the center carb (idle only) on a known good working engine. If I can't get slow idle using the carb it's probably the problem. Either way, I will take the good engine carb and put it on the mustang for an idle only test. If it will idle slow that confirms the intake isn't leaking. (I know it seems like a lot of work but it's actually pretty easy to swap carbs as a test and it eliminates and verifies a lot).

I am writing all of this for my own benefit, both in thinking it through and in 5 years when I have troubles again I can refer back to this thread!

Okay, pulled the carbs. Nothing obvious on the outboards, the plates are closed as far as they can be.
I went to the hardware store and got 4 rubber plugs big enough to fit into the holes for the outboard carbs.
Put the center carb back on.
Car would not start and when I got done cranking I could hear a hiss. (car usually always starts, it's never an issue)
A few pumps of gas, a bit of almost starting and stumbling and one mild backfire that popped two of the plugs out.
I INCREASED the idle speed, turning the screw far enough I could see the throttle linkage start to move.
Engine started, kept running. A little bit rough but it's cold and no choke. I had to turn the idle speed up a bit more.
After warming up for a couple of minutes I was able to turn the idle down, in park, to 700 RPM (via timing light RPM). I didn't try any lower.
No diesel when I turned it off.

I think the HISS I heard at first when it wouldn't start was air trying to be drawn in through the carb, the throttle plates were basically closed all of the way.
So it would appear to be an issue with one or both of the outer carbs. I am going to put one back on and see what happens, then the other.
I might even try to idle it with just the outboard carbs on to see what happens. They don't have an 'idle circuit' but it is my understanding they do allow a small amount of air (there are holes in the butterfly's) so that fuel can be drawn in from the bowls to keep it from 'getting old'.

Can you post a couple pictures.

Might help us see something

The more eyes the better

See pics. One is with the carbs on, no linkage connected. The other is with the rear carb off and 'plugs' in place of the carb.
My next steps after successful idle with just center carb:

Put one outboard carb on in the rear (easiest). Car idled a bit higher, I did not try to adjust so that it wouldn't get too warm. I COULD kill the engine by putting my hand over the center carb. I could actually feel air flow, which I don't recall from before when both carbs were on.
Put on the other carb in the rear and same results. Actually turned down the idle to about 750. It did not try to run on (diesel) when I turned it off.
Put the second carb onto the front, plugs in the rear and same results.
Put rear outer carb on and started. 780RPM, if I cover the center carb the engine dies.
Let it run for a bit and was able to turn down RPM to 600 in park, it was a bit rough but that's not a surprise.
Still able to now kill it with my hands over center carb.

So it seems better. Since I couldn't kill it before with my hands over the center carb I am thinking the air (and fuel) was getting pulled from the outer carbs.
One of two possibilities: 1. When I closed off the center carb completely the air was forced to get pulled from the outer carbs, this seems unlikely. I will test this later or 2. One or both outer carbs was not really shutting all of the way. I know I 'tested' this but I don't recall if I tried it without the linkage

I am also going to try turning the center speed idle all the way back down
I am going to try opening the outer carbs (with no linkage) a little while at idle and see what happens.
Then I start putting it back together. Linkage first and then one at a time hook up the vacuum operated stuff.

Try making a gasket out of cardboard for the outer carbs. No holes for air flow. Don’t hook up the fuel or throttle.

Just work on the center carb. See what you get.

Was this working fine before you worked on the accelerator pump?

If so, what all did you touch fixing the accelerator pump?

The rear carb was just barely 'catching' on opening by hand (no linkage). I messed around with this for a couple hours before I finally swapped it to the front. It doesn't catch in the front and the other carb doesn't catch in the rear. Must be just a bit of difference between the two. I don't keep track of which one was where, perhaps the one was staying open slightly? I may never know.

And after some more fiddling I was a bit surprised at how little either outboard opening increases the idle speed. Disconnected from their physical linkage to the center carb even a small movement would increase the idle speed.

The linkage is adjustable, it's meant to make sure the outboard are closed at idle and don't accidently remain open.

So I think I will need to always readjust and check the linkage anytime I remove the carbs and it would appear, anytime I adjust the idle speed at all. The outer carb linkage cannot keep the center carb from opening but incorrectly adjusted it can hold it open. Too loose and the outboards can open a little on their own.

So I have it all back together. Idle is about 750 in park after it warms up. Vacuum is almost 15 and steady. Now I get to go through the process of tuning it again.

will e said:

The rear carb was just barely 'catching' on opening by hand (no linkage). I messed around with this for a couple hours before I finally swapped it to the front. It doesn't catch in the front and the other carb doesn't catch in the rear. Must be just a bit of difference between the two. I don't keep track of which one was where, perhaps the one was staying open slightly? I may never know.

And after some more fiddling I was a bit surprised at how little either outboard opening increases the idle speed. Disconnected from their physical linkage to the center carb even a small movement would increase the idle speed.

The linkage is adjustable, it's meant to make sure the outboard are closed at idle and don't accidently remain open.

So I think I will need to always readjust and check the linkage anytime I remove the carbs and it would appear, anytime I adjust the idle speed at all. The outer carb linkage cannot keep the center carb from opening but incorrectly adjusted it can hold it open. Too loose and the outboards can open a little on their own.

So I have it all back together. Idle is about 750 in park after it warms up. Vacuum is almost 15 and steady. Now I get to go through the process of tuning it again.

Click to expand...

Cool.

You may need to have a spring for each carb