24in of vacuum at idle too high for the 2.8L?

[SgtDan94]

Can you post some pictures of the carb from the sides or back?

The mixture control solenoid is just that, it is not the same as the Idle Air Control Valve which does not effect mixture, only controls idle speed by allowing air to flow around the throttle plate.

It is almost guaranteed that there is an air/vacuum leak feeding air into the intake and bypassing the carb. This could be almost any device on the vacuum diagram, you really have to check them one by one. I used to have a scanned document of all the vacuum devices used on Fords, and will try to find it.

here are some pictures of the carburetor. Could it be that the mixture control solenoid is letting air bypass even though the engine is already warmed up?? And could it be doing this because the coolant sensor is faulty and is reading cold all the time ?

 

[8thTon]

here are some pictures of the carburetor. Could it be that the mixture control solenoid is letting air bypass even though the engine is already warmed up?? And could it be doing this because the coolant sensor is faulty and is reading cold all the time ?

Looking at the 2nd, 3rd and 4th pictures it appears the idle speed is in contact with the stop. This is an electrically controlled solenoid/actuator which is either actively controling the idle speed or is a one position anti-diesel solenoid - I believe it is a speed control on yours but would have to look into that further. In either case when the engine is off it will allow the throttle plates to go almost or completely closed to prevent run on (dieselling). But when powered it will prop the throttle plates open.

 

[SgtDan94]

Looking at the 2nd, 3rd and 4th pictures it appears the idle speed is in contact with the stop. This is an electrically controlled solenoid/actuator which is either actively controling the idle speed or is a one position anti-diesel solenoid - I believe it is a speed control on yours but would have to look into that further. In either case when the engine is off it will allow the throttle plates to go almost or completely closed to prevent run on (dieselling). But when powered it will prop the throttle plates open.

When i took these images the throttle was in fact in contact with the solenoid stopper thing... i believe it only ended up this way because of when i last turned it off.

I do know for a fact that even without the solenoid touching the throttle, it still idles with the idle screw backed all the way out. In other words, the solenoid is not the reason the throttle wont close. I moved that solenoid all the way back using its adjusting screw and unplugged it while i was setting the idle, so that it would not interfere. With the driver side idle adjust screw completely removed, there is still about a 1/4 in gap between the part of the throttle that comes in contact with the screw, and the idle screw tab itself.

I apologize for wording things in a confusing manner, i lack proper terminology

 

[SgtDan94]

@8thTon

Here is an image with captions that better explains what im trying to say. Again, we can disregard the solenoid that moves the throttle since i tested it with the solenoid removed.

 

[SgtDan94]

@8thTon @Uncle Gump

Would an incorrect power valve cause high idle? I remember the one i installed that came in the rebuild kit looked just slightly different than the one i removed. . and the one i removed was in decent/ good shape.

 

[8thTon]

@8thTon @Uncle Gump

Would an incorrect power valve cause high idle? I remember the one i installed that came in the rebuild kit looked just slightly different than the one i removed. . and the one i removed was in decent/ good shape.

Probably not, unless it leaked air in around the cover, etc. It is using manifold vacuum to hold the power valve closed. Some of these have external passages to the manifold with a rubber tube to the cover, some use internal passages. Regardless it’s a dead end run to the intake. If the diaphragm leaks then you suck fuel directly into the intake, which is usually quite noticeable. If the valve opens too soon or is stuck open it adds fuel to the main fuel booster circuit, but at least it is still controlled by a jet. So bad power valves usually cause excessively rich mixture rather than air leaks.

 

[Uncle Gump]

I think 8thton hit the power valve operation.... when they go bad... they basically dump excess fuel when not needed a run really rich.

I didn't know this had an idle speed actuator... judging by the clean threads on it's adjuster mechanism you have adjusted it. The throttle is sitting on this actuator thus not allowing the throttle to rest on the actual idle speed screw. So I ask... does the speed actuator bump the idle speed up when turning on the air conditioning? Either way... I would back off that adjustment with it unplugged until you can control the base idle speed with the actual idle speed screw. Get that to spec then plug in the actuator and set that adjustment to bump the idle up a little with the A/C comes on.

Test your vacuum gauge on another vehicle... I still have a feeling your vacuum gauge isn't very accurate.

 

[8thTon]

The throttle plates seal in the bore quite well if they close without a stop, and there is no way the engine can run like that unless the air is getting into the intake through some other path - but if it were then it would pick up no fuel.

So unless you have some very unlikely combination of things like the power valve is leaking just enough fuel to make up for a vacuum leak, then there is some piece of info that’s missing. To idle properly there must be enough air going past the edge of the throttle plates to activate the idle fuel circuit. So the plates cannot be fully closed.

In normal operation they are fully closed when the engine is off, but then the idle speed actuator opens them and controls the idle speed.

 

[SgtDan94]

The throttle plates seal in the bore quite well if they close without a stop, and there is no way the engine can run like that unless the air is getting into the intake trough some other path - but if it were then it would pick up no fuel.

So unless you have some very unlikely combination of things like the power valve is leaking just enough fuel to make up for a vacuum leak, then there is some piece of info that’s missing. To idle properly there must be enough air going past the edge of the throttle plates to activate the idle fuel circuit. So the plates cannot be fully closed.

In normal operation they are fully closed when the engine is off, but then the idle speed actuator opens them and controls the idle speed.

@Uncle Gump



I understand. This is a motorcraft carburetor but it’s been remanufactured by Holley, according to the sticker. I removed and unplugged the actuator and it made no difference... still idling normal fully closed... not touching the adjustment screw.

I am certain that the previous owner messed with the carburetor and couldn’t get it running himself, since, when I picked it up from him, he said it needed a new carburetor to run... it didn’t however.

I’m thinking that the throttle plates were removed and replaced and are not seated correctly.. this has to be the reason. I’m confident that there are no other vacuum leaks... I replaced all vacuum lines and the carburetor gaskets between the intake. It holds a very stable vacuum pressure, wether it’s 20 or 24 HAHA depending on the gauge. There’s not much to this engine, as you guys know of course. I’m going to remove the carburetor again and flip it over.. see what’s really going on. I will take some pictures and share updates.

I will also be replacing a leaky mechanical fuel pump and failing oil sending unit. I’m ready to SMOG and drive this bronco.

 

[8thTon]

@Uncle Gump



I understand. This is a motorcraft carburetor but it’s been remanufactured by Holley, according to the sticker. I removed and unplugged the actuator and it made no difference... still idling normal fully closed... not touching the adjustment screw.

I am certain that the previous owner messed with the carburetor and couldn’t get it running himself, since, when I picked it up from him, he said it needed a new carburetor to run... it didn’t however.

I’m thinking that the throttle plates were removed and replaced and are not seated correctly.. this has to be the reason. I’m confident that there are no other vacuum leaks... I replaced all vacuum lines and the carburetor gaskets between the intake. It holds a very stable vacuum pressure, wether it’s 20 or 24 HAHA depending on the gauge. There’s not much to this engine, as you guys know of course. I’m going to remove the carburetor again and flip it over.. see what’s really going on. I will take some pictures and share updates.

I will also be replacing a leaky mechanical fuel pump and failing oil sending unit. I’m ready to SMOG and drive this bronco.

Yeah if they are not centered you should be able to see that easily.

 

[Uncle Gump]

@SgtDan94

I have a feeling things are getting lost in the communication here. So here are my thoughts...

1. I think what we are calling the idle speed screw is actually an idle stop screw. This would be set during the rebuild perhaps with a feeler gauge or lettered/number drill bit to a clearance between the throttle plates and throttle bores. Or maybe an RPM spec once running with the actuator unplugged. Once it is set... you don't fool with it again. I'm certain there is a proper spec and procedure for setting this in the service manual. I just don't have one... and it's been probably 3 decades since I and most of us have really fooled with carbs of this nature... I know I'm rusty with all the details. If you don't have a manual... you should invest in one. A factory manual will far outshine the others that are out there. I would think you could get an electronic version on ebay.

2. The idle control actuator is actually what controls the idle speed. When you turn the key ON that actuator should bump out and contact the throttle lever. This will control your idle speed. This is adjusted with it's own adjusting screw. The one with the spring in the last picture you posted. I have a feeling what you have here is that idle speed actuator isn't working. I can tell by the clean threads that it was turned out and is contacting the throttle lever in your picture. Most likely it has been adjusted out the same distance it would bump out if it was working properly. So with the key OFF... there should be a space between the idle speed actuator plunger and the throttle because the idle speed actuator is retracted... and the throttle would be in contact with the idle STOP screw. Again... a service manual would be worth more than another thousand words typed in this thread.

3. I hope this makes sense.

4. Did I forget to mention you need a decent service manual? It will seriously save you some headaches.

 

[SgtDan94]

@SgtDan94

I have a feeling things are getting lost in the communication here. So here are my thoughts...

1. I think what we are calling the idle speed screw is actually an idle stop screw. This would be set during the rebuild perhaps with a feeler gauge or lettered/number drill bit to a clearance between the throttle plates and throttle bores. Or maybe an RPM spec once running with the actuator unplugged. Once it is set... you don't fool with it again. I'm certain there is a proper spec and procedure for setting this in the service manual. I just don't have one... and it's been probably 3 decades since I and most of us have really fooled with carbs of this nature... I know I'm rusty with all the details. If you don't have a manual... you should invest in one. A factory manual will far outshine the others that are out there. I would think you could get an electronic version on ebay.

2. The idle control actuator is actually what controls the idle speed. When you turn the key ON that actuator should bump out and contact the throttle lever. This will control your idle speed. This is adjusted with it's own adjusting screw. The one with the spring in the last picture you posted. I have a feeling what you have here is that idle speed actuator isn't working. I can tell by the clean threads that it was turned out and is contacting the throttle lever in your picture. Most likely it has been adjusted out the same distance it would bump out if it was working properly. So with the key OFF... there should be a space between the idle speed actuator plunger and the throttle because the idle speed actuator is retracted... and the throttle would be in contact with the idle STOP screw. Again... a service manual would be worth more than another thousand words typed in this thread.

3. I hope this makes sense.

4. Did I forget to mention you need a decent service manual? It will seriously save you some headaches.

What you’re saying makes sense. Unfortunately this bronco has probably been messed with by a few different people since it was last registered in 1997. For that reason it is all out of wack and I’m simply trying to bring it back into working order. I know a proper manual is key.

I know for sure, 100%, that the bronco idles fine with the control actuator completely removed, and the throttle all the way closed.

when the actuator is installed, it works. I can see it moving and it sometimes keeps the engine from dying. I especially see it work if I give it some throttle by hand... when the RPMS come back down, the actuator kicks the throttle a little to keep it from dying .

 

[Uncle Gump]

OK... so it does work... and it's a stepper motor... not just an actuator.

Gotta be alignment of the throttle plates or the throttle shaft is twisted. Is the throttle shaft tight in the bore?

I think if it runs a drives good... I would turn the idle stop screw out until it barely touches the throttle lever... and just drive it... SMOG it and see what you have.

 

[19Walt93]

If you think the throttle plates have been removed check them very carefully. The original screws are swedged to keep them from backing out and falling into the engine.If you remove the crews to center the plates in the bores I'd recommend a drop of locktite on each one when you assemble it. The base idle screw is not a throttle stop, it's one of the tune up adjustments. If the screw is backed out of contact with the throttle lever and it still runs, there's a problem. The idle control isn't a stepper motor, it's just an actuator but some have one position controlled electrically and one by vacuum.

 

[8thTon]

If you think the throttle plates have been removed check them very carefully. The original screws are swedged to keep them from backing out and falling into the engine.If you remove the crews to center the plates in the bores I'd recommend a drop of locktite on each one when you assemble it. The base idle screw is not a throttle stop, it's one of the tune up adjustments. If the screw is backed out of contact with the throttle lever and it still runs, there's a problem. The idle control isn't a stepper motor, it's just an actuator but some have one position controlled electrically and one by vacuum.

Yeah, I use LocTite red for throttle plate screws.