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


SgtDan94

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I did some research on how to adjust the 2150 carburetor on my 1984 Bronco II using a vacuum gauge. Every where I read, they said 18-20in of vacuum is normal for a healthy V6...

I was able to pull 24-25in stable on the gauge while idling at a normal RPM.... is this ok??

I would like to add that the curb idle screw is ALL THE WAY backed out and it’s idling just fine.. normal.

I know that In general you should aim for the highest possible vacuum pressure but 24-25 seems over and above what anyone else is getting. I installed a T in one of the vacuum lines coming off the manifold tree for my vacuum gauge.

My bronco has 150,000mi and is in very good shape, probably driven lightly for most of its life. I’m planning on just leaving it this way.. unless this high of vacuum pressure is a bad thing.
 


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Uncle Gump

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It could be that your vacuum gauge is off... not calibrated. If you don't have idling issues... or drivability symptoms... I wouldn't worry about it. Maybe check another vehicle with your gauge and see what it shows.

I know these to have an idle speed screw and a fast idle speed screw. The terminology may differ depending on who you're talking to. Idle speed screw would contact the throttle lever and the fast idle speed screw will contact the fast idle cam... which is part of the choke system. If you can back the idle speed screw all the way off and it is not contacting the throttle lever... and the engine still runs... you may have bent throttle shaft or the shaft bushings are worn. Does the throttle shaft have movement in the shaft bore? Did you remove the throttle plates from the shaft?
 

SgtDan94

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It could be that your vacuum gauge is off... not calibrated. If you don't have idling issues... or drivability symptoms... I wouldn't worry about it. Maybe check another vehicle with your gauge and see what it shows.

I know these to have an idle speed screw and a fast idle speed screw. The terminology may differ depending on who you're talking to. Idle speed screw would contact the throttle lever and the fast idle speed screw will contact the fast idle cam... which is part of the choke system. If you can back the idle speed screw all the way off and it is not contacting the throttle lever... and the engine still runs... you may have bent throttle shaft or the shaft bushings are worn. Does the throttle shaft have movement in the shaft bore? Did you remove the throttle plates from the shaft?
I did not remove the plates and the shaft does not have play. It’s possible that someone else did?? Should the throttle make contact with the tab that the idle screw threads into.. with the screw all the way out? Because it doesn’t..

It idles and drives ok. I don’t have a tachometer installed, though I plan to install one (posted a different thread about it). It idles pretty low and seems to run smooth.

Here’s the thing.... I got this bronco a few days ago and it wasn’t running, the previous owner said “it needs a new carb.” Instead, I took the existing carb off, a motorcraft that was remanufactured by Holley probably back in the 90s, since this Bronco was last registered in 1997. The carb looked clean inside... like it didn’t need a rebuild and instead just needed to be calibrated. So I reset the mixture pins to 1 1/2 turns out, and I had to turn IN the WARM idle screw in order for the engine to idle with pins at 1 1/2 turns....

From there, I adjusted the pins until the vacuum reached 24in... it wouldn’t pass 24in, but had a steady/ stable needle. I then proceeded to turn the idle down... I ended up unscrewing the idle screw (on the drive side) all the way and it just so happen to sound correct (by ear) once it was all the way out.

to be clear, the tab that the “warm idle” screw threads into... the throttle doesn’t come into contact with this tab when the screws all the way out... I don’t know if it’s supposed to, but there’s about a quarter inch of space in between
 
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Uncle Gump

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I'm confused with the term warm idle pin... what is that?
 

SgtDan94

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I'm confused with the term warm idle pin... what is that?
I corrected “pin” for screw.. I’m referring to the idle adjusting screw on the driver side. The screw you would adjust once the engine is at operating temperature. Not the fast idle screw. I understand the fast idle screw is only relevant while the choke is engaged.
 

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It's probably normal for there to be a gap if the idle speed screw is backed out all the way. Again... I wouldn't worry to much about that... unless the idle speed is way to fast. Or if you can physically close the throttle more... in that case I would put a return spring on the throttle.

It sounds like you're on the right track...

I would try putting some miles on it... adjust as necessary. You may have to tinker with it a bit to get it right.
 

SgtDan94

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It's probably normal for there to be a gap if the idle speed screw is backed out all the way. Again... I wouldn't worry to much about that... unless the idle speed is way to fast. Or if you can physically close the throttle more... in that case I would put a return spring on the throttle.

It sounds like you're on the right track...

I would try putting some miles on it... adjust as necessary. You may have to tinker with it a bit to get it right.
Im getting a SMOG pretty soon here. Im interested in seeing the results. I have little experience with carburetors so I would like to see if I at least came close to calibrating it’s correctly.

honestly, I think the idle can be increased a little bit.. I’ll know once I figure out how to get the tach installed.
 

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Idle is always the dirtiest... the lower it idles the worse it gets. Running a few extra rpms during a smog test will not hurt you one bit. Just as long as it doesn't exceed the idle speed thresholds of the test.
 

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Having a small vacuum leak can help smog too.
 

SgtDan94

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Having a small vacuum leak can help smog too.
Idle is always the dirtiest... the lower it idles the worse it gets. Running a few extra rpms during a smog test will not hurt you one bit. Just as long as it doesn't exceed the idle speed thresholds of the test.
Speaking of vacuum leak.. There’s a vacuum line that stubs up within the intake.. and within the circular air filter, beside the circle cutout for the carburetor. Should this have something attached to it? (See attached)
 

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Uncle Gump

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It's been to many years since I've looked at one of these... I think that might be a thermo switch for the air pre heat door in the snorkel. I don't believe there is anything connected to it from the inside. @19Walt93 worked with these and has a super memory... he might know for sure.

As for the truck being prewired for a tach... I'm not sure about that. I kinda thought a tach was standard equipment with a manual transmission. Tach hook up is pretty basic... power, ground, illumination and negative side of the ignition coil.
 

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Your throttle plates should not be closing so far that they bind in the throttle bores, they are supposed to hit a mechanical stop (idle speed adjust). If your idle is too high then you likely have a vacuum leak somewhere. If it's small you may be able to tune it in terms of mixture, but it will still add air and increase idle.

Sometimes a hot idle device is added, which is a small bimetal thermostatically activated air leak intended to increase idle speed on hot engines. This could be mounted in the air filter housing. It looks like what I'm seeing in your picture, but it's hard to tell. If that has failed open then it would be your high idle problem. Plug it and see what happens, it's no big deal to run without it for a while. If you plug it disconnect the hose and plug it at the line, covering that exposed tube won't work.

A larger tube may run into the air filter which could be the fuel bowl vent. That would not have anything on it.
 

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The hot idle compensator isn't in the air cleaner, it would mount to the back of the carb. The piece in the air filter shouldn't be capped. Vacuum should feed one side of the vacuum valve and the other side would connect to the air cleaner vacuum motor that allows warm air to be drawn into the air cleaner during warm up.
 

Uncle Gump

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I kinda thought that is what it is... Thanks for the clarification Walt.
 

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@SgtDan94 ...

Something else I noticed in your picture... The air cleaner snorkel is missing the flexible coupler between the air cleaner and the cold air intake. This would allow heated air in the engine compartment to be drawn into the air cleaner opposed to drawing in fresh air from the front of the vehicle. You should look into sourcing one of those...
 


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