Real time help adjusting idle mixture

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Nov 19, 2005
Pasadena, CA
I'm trying to perform the drop lean using a vacuum gauge.

I hooked the gauge up to the brake booster line and getting about 14" of vaccuum

I've read the how to,

and hope I got this right,

lower the idle to around 450-500

and THEN

open the choke <---- does that mean pull out the choke all the way?

and then adjust the idle mixture screw until I hit a plateau and then back off a little bit.

then adjust idle back up to spec.

problem is no matter what I do to the idle mixture screw, it doesnt affect the vacuum or the idle or anything....

I backed it out all the way and in all the way. the gauge is bouncing around so its hard to read but I cant perceive any difference from adjusting the screw... Am I doing this right?

I checked for any vacuum leaks by spraying water , spraying wd40, covering the carb and killing the engine looking for a rpm increase before dying. but everything seems to be okay.

Not sure about the 14" of vacuum tho, seems a bit low
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Here's a link to a post with the lean drop method from the FSM. The engine should be at operating temperature. The choke should be open, meaning the knob on the dash is pushed all the way in. If your air cleaner is removed, it should be easy to tell that the choke (the plate at the top of the primary throttle bore) is open (ie. not restricting air flow into the carb).

IH8MUD Forums - View Single Post - carberator

I start with the idle mixter screw (the screw on the side of the carb next to the valve cover) unscrewed about two full turns for fully seated. Start the engine and adjust the idle speeed screw (see pic in other post) so the rpm is about 690 (I'm not sure how you do this with a vacuum gauge). Then you adjust the idle mixture screw unitl you maximize the rpm (I guess with a vacuum gauge you would try to maximize vacuum). Once you've found the local maximum, re-set the idle speed screw to 690 and repeat the idle mixture adjustment until the rpm (or vacuum) no longer increase. When this is completed then you drop the idle speed to 650 by screwing in the idle mixture screw (hence the name "lean drop"--as you turn the screw in to drop the rpm, you are leaning out the idle mixture).

Since you need a tachometer to do this job anyway, I don't know why you need to use a vacuum gauge. But if you do, connect the vacuum gauge to manifold vacuum. A good source is the vacuum source for the AC Idle up vacuum diaphragm (if you have AC). This is located at the back of the intake manifold. It's a finned connector that screws into the manifold with a large port going to the brake booster and a small port going to the AC idle up.

Make sure you are turning the right screws (see drawings on the linked post above). The idle mixture screw is pretty obvious--it's the only screw on the valve cover side of the carb. However, from the factory these came sealed with a plug of lead. If yours still has the lead you will have to drill it out to access the screw. The idle speed screw is on the firewall side fo the carb tucked in next to the base. Don't confuse it with the fast idle screw, which is used to adjust the fast idle when the choke is closed.

edit: this thread has a post with an attached PDF. On the second page of that PDF is the schematic for the AC idle up showing the vacuum source connector.
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It makes a difference with Vac reading if you do the lean drop with the Air Cleaner on... Hard to get the the mix screw, but I do it with a long thin blade and a flashlight and lots of cursing.

edit: FSM procedure calls for Air cleaner on.
I can't seem to make the adjustments with the air cleaner installed, but when I take the air cleaner off, I leave the HIC valve connected and just move the air clearer out of the way.
And some people do it with their hand or a piece of cardboard over the carb to simulate AC flow restriction.
I stuck a stubby screwdriver behind the carb and was able to adjust the mixture screw with the aircleaner installed.

Only problem was the mixture didnt do anything to the vacuum.

I'm only at 14-15" and checked for leaks but couldn't find anything.

Would 14-15" of vacuum suggest I have a vacuum leak?

I did however find that I'm missing a bolt from the j pipe.:rolleyes:

and also is it safe to drive after messing around with the mixture screw ? I could be too lean, but this only affects idle right?
The idle mixture screw only sets the air fuel mixture at idle. There are other circuts in the carb that maintain proper mixture during cruise, power-load, acceleration, etc. If it's idleing OK and you're not idleing for hours on end you should be OK.
14 to 15 inches of vacuum is too low. Check your valve adjustment and search for leaks. The fact that no change is evident at any idle mixture setting and the fluctuating vacuum reading are also indicative of a problem.
Thanks guys , I guess its time to hunt down the gremlins:mad:
Figured this might be the best thread to use to figure out what is going on with my carb when I try and do the lean drop adjustment on it. Here is what I did and what happened:

Started out with the idle mixture screw at 2 turns out from fully seated.
Adjusted idle speed screw to 690
Adjusted idle mixture screw up to maximum, which was really high, around 1300 RPM's (was probably a good 5 or 6 more turns)
Went to adjust idle speed back down but it wouldn't drop below 950 RPM's or so.

I tried this with the air cleaner on and off, and off with some paper restricting airflow, and it did the same thing every time. What do I need to look at to get this adjustment done correct?

Since I failed high HC's at idle on my smog test, I assume I need to lean out the mixture more. Is the idle mixture screw lean when screwed in all the way? I would assume so, but want to make sure of that as well.

Thanks in advance. :cheers:
5 or 6 full turns out from 2 full turns out is too much, in my opinion. I adjust the idle mixtue screw in 1/4 turn increments and let it stabilize after that. If the RPM's gets up to about 1000, then I'll adjust the idle speed screw to bring them back down to 690, and the go back to the idle mixture screw.

Are you sure you're really turning the idle speed screw and not the fast idle speed screw?
I put this on hold while I was working on some other things but am back to this being the last thing I need to do before I go and try and get smogged again.

I made sure that I was adjusting the right screw after the first couple times that it didn't seem to be working right. I know awhile ago when I was just messing with the idle I had turned the fast idle by mistake. But I don't think that would affect the rest of the adjustments.

I just want to try and have the best mixture I can without being too lean or rich as either way will mess up my numbers. Just wanted to figure out if maybe there was something wrong that was keeping me from getting the adjustment to work right and if anyone has an advice on what to try.
Have you triple checked that your vacuum lines- especially the ones to the carb, are connected properly? The 4 lines on the valve cover side of the carb were out of order for me and I had trouble setting the idle until I got those corrected.

The other thing would be make sure the carb linkage is operating smoothly and not binding, keeping the idle from returning to normal.
Have you triple checked that your vacuum lines- especially the ones to the carb, are connected properly? The 4 lines on the valve cover side of the carb were out of order for me and I had trouble setting the idle until I got those corrected.

The other thing would be make sure the carb linkage is operating smoothly and not binding, keeping the idle from returning to normal.

I did have the binding issue but got that all fixed up. The spring was missing from the accelerator and then I also greased up the linkage.

As for the vacuum hoses for the carb, I'm going to double and triple check that this evening. Thanks for that tip.

Any other things I should look at?
Checked everything this morning for the fourth time and tried the lean drop again, and same result. The mixture screw is way out. I'm scheduled for a smog check this afternoon and just don't want another failure and another $60 gone. Unless anyone has some advice on what I can do to get this set right, I'm just going to go with what I've got and hope it all goes well. Taking some tools with me so maybe they will let me adjust stuff as I go. We'll see what happens I guess.
my truck seems to have multiple idles...if I tap the gas it will be high but if i lift it a little with my foot, it will go low. Should I try spraying some lube on the assembly? or is this normal. I cant seem to figure out which is the true idle, so now it feels like my truck runs worse than it did before.
Yeah- you better check your linkages for binding, debris, etc. Mine idles very consistently.
The linkages should not have to be luburicated, but it should clean and rotate freely. Also check that you have both return springs. There is one that is on the carburetor, and another on the pedal.
carb linkage.JPG

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