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Discussion Starter #1
I'm new to the forum and just picked up a '79 w/351m a month or so ago.

I'm having issues getting my low idle dialed in the way I think it should be with my carb...

The P.O. added a 4barrel Edelbrock performer, aluminum intake, & headers. Since it had been in storage for awhile prior to when I picked it up, the first thing I did when I got it home was clean the carb (it wasn't idling well before I cleaned it either).

I inspected the intake => body assembly gasket, and body => airhorn gasket, all look good and are sealing well.

I also replaced the step-up metering rods, just because on dis-assembly they showed signs of corrosion.

Finally, I gave everything visible a good scrubbing. I did NOT tear down further than previously described.

In general, it runs MUCH better than it did before I cleaned it. It's smooth at mid-full throttle and response is good. However...

When I am idling anywhere, it always wants to stall out and die. It is especially bad when it's still cold, after the electric choke is fully open and warm. To combat this and make it driveable, I had to turn up the idle rpm screw to compensate. I don't like this as it's much higher than I feel it should be, and causes abrupt and harsh shifting from P/N into R or D.

I have tried, tried, and tried again adjusting the low idle mixture screws in accordance with the manual, youtube, etc... By cranking them down 1 at a time until the engine sounds like dying, then backing out until it gets smooth again. However it doesn't make my problem go away.

I got to looking at the carb and noticed that 'ported vacuum port' has a elbow connector on it, but no line hooked up to it. (see pics) Could this be my issue? What is its function? Any other ideas?





This is the only issue keeping me from considering my engine to be running 'reliably' so I want to figure it out asap.

Thanks!!
 

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I'm no guru, but I think it may be a little lean at idle. Why lean because you say it's worst when the engine is cold. Cold start requires a mixture leaning just a bit to the rich side.

Do the mixture screws do anything? I'll assume they do. If so I think I would look at some of those vacuum hoses for small cracks. Maybe even mark them, then remove same. Plug ports at carb and check your idle for improvement.

Don't drive it like that as one of those hoses could be for power brake booster. Should not be but could be.

If it helps check hoses and items attached to same.perhaps one at a time.
 

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Ported carb should go to a vacuum advance distributor. Technically that shouldn't affect idle as it only comes into play outside of idle slot, but what your explaining does sound like a vacuum leak somewhere. Plug that port for the hell of it. Spray some carb kleen around the intake to head interface and the base of the carb and around the throttle rods going into the carb if the things dies off u have a vacuum leak at that point. If no vacuum leak then its onto the carb internals and set up i think. I have only holley experience so tuning the edelbrock will be some one elses info. Hope that helps some
 

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You have a vacuum leak most likely. Put a vacuum gauge on the intake manifold and see what it is doing.
 

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Discussion Starter #5
Thanks for the ideas! I will definitely try the carb cleaner trick when I get a chance.

You have a vacuum leak most likely. Put a vacuum gauge on the intake manifold and see what it is doing.
Pardon my ignorance but could you explain this a little more? I've never dealt with a vacuum problem before. Is there a certain port I should be looking for on the manifold to hook up to? Is there a certain value I should be looking for on the vacuum gage, or just a pattern of behavior? Also, I'm guessing the vacuum gage itself is something I could pick up at an Oreilly's?
 

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the one that is open should go to the distributor. That big hose should be for the PCV valve and the smaller one on the right is manifold vacuum.

You should also have a vacuum tree at the back of the manifold which goes to a few things or is blocked off. But it defiantly should go to the brake booster.

Since you said your having issues tuning by ear, my guess is there is a vacuum leak or the jets are plugged up.

You also can tune the idle mixtures from idle vacuum.
 

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Agreed with everyone else. That's the sign of a vacuum leak.

If you don't have a good vacuum gauge, get one. They're the best engine tuning tool you can ever have. Your engine is a big air pump, so knowing the vacuum can tell you a great deal about its health.

Warm up the engine and have it idle. Put your vacuum gauge on direct manifold vacuum.

Hold the vacuum gauge in one hand and a can of carb cleaner in the other and spray generously around intake ports and the base of the carb. At any time you see the needle on the vacuum gauge dip and waver, you've found your leak. Fix that before proceeding to anything else.

Also, simply make sure that any and all vacuum ports in the engine that are NOT in use are fully sealed and capped off.

Once you've eliminated all vacuum leaks, adjust the idle mixture screws until your engine has the highest vacuum you can get with the smallest amount of opening on the screws.

In other words, turn the screws in until the vacuum starts to drop. Then turn the screws out slowly until you get the highest vacuum reading you can get. Then turn them back in slowly as far as they will go without changing the vacuum.


Next, set your timing with a vacuum gauge. Disconnect the vacuum advance on the distributor and cap it off at the carburetor. Loosen the adjusting nut on the distributor and advance/rotate it until you get the highest vacuum reading possible. Now retard/rotate the distributor until the vacuum drops to about 1 to 1 1/2 hg below the highest reading. Tighten it down and reconnect the vacuum advance.

Your engine should be well tuned up. (But keep in mind the tuning will only work right if all vacuum leaks are eliminated first.)

Good luck!
 

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Also be sure the vacuum booster for the brakes has not failed. This thing can cause you a huge headache of a vacuum leak.

As the same goes for the heater / defroster stuff, they are ran from vacuum.
 

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Discussion Starter #10
Thanks again for all the help! Got some good ideas. So I was able to pick up a vacuum gage and play around a little with it today... I may have confused myself more unfortunately. :doh0715:

So to start my diagnostics I hooked up the vac gage to the tree on the rear of the manifold, and hooked up my multimeter to read rpm.

The first thing I noted was that the distributor advance vacuum line was hooked up to the far right 'full time manifold vacuum' port, and the big line appeared to be hooked up to the valve cover PCV valve properly, and the left ported vacuum port was plugged. See picture.



I started her up and let her get nice and warm. I noticed that I was pulling ~18inHg at ~15-1600 rpm.



I then did the carb cleaner trick, and never noticed the vacuum drop, but actually increase very slightly along with engine rpm to about 20inHg when I sprayed liberally directly on the left hand side of the carb on the the rearmost rotating mechanism (connected to choke??) cant find it in the exploded views... But i was not noticing anything when i sprayed directly on the gasket areas. The vacuum increase with increase rpm is what I would expect though.

I also sprayed the tree, line to brake booster, distributor advance, etc...

I decided it seemed like i had good vacuum and that was not the problem... at least yet. I then tried playing with the idle mixtures and could never get it to live below 1000rpm... what kind of low idle speeds are you guys running? because i cant seem to get low at all...

I also did play with the vacuum connections in general. With the ported vacuum hooked up to distributor advance, pcv hooked up, and full time vacuum hooked directly into manifold tree, i noticed no significant impact. Then I plugged the full time vacuum and noticed little impact as well. There was no existing hookup for the full time vacuum port other than to the tree.

SO now i'm really confused. Could it realistically be a vacuum issue if I'm pulling 18inHg? I didn't mess with the timing at all yet, could that be it? Spark plugs? I did notice that my gaps look pretty big and I got some to replace them with but haven't put them in yet... :banghead

I'm wondering if I don't have a big chunk of something stuck somewhere in the carb? It does seem like my mixture screws have very little impact on anything I do...

Here's hoping for a revelation of some kind!!!
 

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A couple more places to check for a vacuum leak;

Looks like a factory air conditioning truck. If it is, all the inside ductwork is controlled by vacuum. There should be a vacuum line off the "tree", that goes thru the firewall, to the climate controls. The climate controls have been known to go bad, and cause a vacuum leak.

Also, attached to the tree, should be a vacuum line that hooks to a steel line that connects to the trans modulator. The tranny end will have a short piece of hose that connects to the modulator. Check that for cracks/splits.

The idle screws won't do much till you get the rpms down.
 

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If the throttle changed when you sprayed on some of the rotating assemblies on the carb, it's possible that you have worn throttle shafts, or the like. Do you know the age of the carb?

Direct manifold vs. ported vacuum can make a difference in your idle RPMs due to more timing at idle with direct. However, direct isn't necessarily a bad thing (ported is more for emissions). I run mine on direct manifold and it idles around 600 - 700 RPMs.

The idle mixture screws don't really change your idle RPM unless they're way out of tune. Did you try your base idle screw? It's attached to the throttle linkage. As you turn it in, it pushes against the carb body and forces the throttle linkage open. As you turn it out, it lets it close and brings it back down.

Also be sure your choke linkage isn't gummed up and not letting it kick off of high idle.
 

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Also be sure the vacuum booster for the brakes has not failed. This thing can cause you a huge headache of a vacuum leak.

As the same goes for the heater / defroster stuff, they are ran from vacuum.
These two areas can cause vacuum leaks which you cannot see. Remove the booster from the manifold tree and plug the tree. see if you get any change.

Trace the vacuum hoses from the heater / defroster and see what they do when plugged.

I would also verify the timing is @ 8-10 degrees btc. Another thing about these engines is they have a tendance for the balancer ring to slip and this will set your timing way off.

Honestly with a carb looking like that, I bet the idle mixture chambers in the carb are gummed up. That is why the idle circuit is not functioning.
 

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The heater/defroster stuff is most likely all cable driven. I know my '81 is.

But yes, the brake booster for sure. One way to check it is to simply disconnect it and cap it off and go for a test drive. Just know your brakes won't be as easy to use, or responsive, so be careful.
 

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Agreed with everyone else. That's the sign of a vacuum leak.

If you don't have a good vacuum gauge, get one. They're the best engine tuning tool you can ever have. Your engine is a big air pump, so knowing the vacuum can tell you a great deal about its health.

Warm up the engine and have it idle. Put your vacuum gauge on direct manifold vacuum.

Hold the vacuum gauge in one hand and a can of carb cleaner in the other and spray generously around intake ports and the base of the carb. At any time you see the needle on the vacuum gauge dip and waver, you've found your leak. Fix that before proceeding to anything else.

Also, simply make sure that any and all vacuum ports in the engine that are NOT in use are fully sealed and capped off.

Once you've eliminated all vacuum leaks, adjust the idle mixture screws until your engine has the highest vacuum you can get with the smallest amount of opening on the screws.

In other words, turn the screws in until the vacuum starts to drop. Then turn the screws out slowly until you get the highest vacuum reading you can get. Then turn them back in slowly as far as they will go without changing the vacuum.


Next, set your timing with a vacuum gauge. Disconnect the vacuum advance on the distributor and cap it off at the carburetor. Loosen the adjusting nut on the distributor and advance/rotate it until you get the highest vacuum reading possible. Now retard/rotate the distributor until the vacuum drops to about 1 to 1 1/2 hg below the highest reading. Tighten it down and reconnect the vacuum advance.

Your engine should be well tuned up. (But keep in mind the tuning will only work right if all vacuum leaks are eliminated first.)

Good luck!
If he's having timing issues those need to be sorted first before adjusting the carb. Should be check for vac. leaks, timing, then carb.

Could be a vacuum leak considering that ported vacuum on the carb has nothing running to it. If it isn't plugged, it "should" idle really high, at least that's been my experience.

If the PO didn't set the initial correctly, he could have an extremely low initial and once it raises in rpm the advance starts working and the engine would run fine.

Bottom line is address each issue one at a time starting with checking for vac. leaks, followed by timing, then tune the carb.
 

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True, if the timing is way out, it's going to throw things off.

Generally, when I'm tuning things up, I'll do multiple passes over everything. Set the idle RPM, idle mixture, adjust timing, RPM, mixture, timing, etc. etc. until none of the adjustments affect the other.
 

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Now that I see pictures and the dizzy that you have, yeah you need to swap vacuum ports. On some holleys there is a expoxied-in plug that sometimes deteriorates in the casting for the choke assembly. I'll assume that edlebrok could have the same thing going on, however the hole in the casting is before the throttle plate, so what you spray is more easily distributed throughout the cylinders vs a leak at the base at one barrel... which would cause a little stumble and the engine to rev.

Whenever you swap the vacuum port that you're dizzy will be hooked to, you will probably have to check your timing. I'm wondering if you've gotten any stumbling on acceleration whenever you've driven it.

Why are you measuring vacuum at 1200-1500 RPM? What is your current idle speed? Have you shined a light into the carb yet to check for varnish?
 

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I guess people are not reading his updates.

he has fixed the vacuum with the dizzy.

As for the high idle it is because it will not run any lower.

That is his main problem right now
 

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Discussion Starter #20
VICTORYYYYYYY!!!!!!!!!!!!!!!!!!!!!!! :armed

I have successfully tamed the beast!!!!

First of all, I have learned more about carburetors, diagnostics, and common sense in the last 48 hours than the first 25 years of my life...

And the winner is: common sense.

After repeating the same checks that I have done before with the vacuum, (even isolating/disconnecting the brake booster circuit) thinking things through, I decided that I could not rule out a dirty low idle carb circuit. I decided to remove, dismantle, and clean every nook and cranny I could get at.

What I ended up finding was TERRIBLE particle/gunk built up in the primary venturi boosters, and bad varnish down in their seats/cavities. :doh0715::banghead:banghead:banghead

After scrubbing and poking at it with small brushes, I blew everything out with carb cleaner & compressed air.

I then reassembled everything painstakingly, and made sure that I used ALL the vacuum ports, and made sure they were hooked up per jopes' first prior post. (None of this plug the port and not use it crap the PO had setup)

I set the mixture screws 2 turns out from closed, and bumped up my idle screw, just for starting/warm up.

I fired her up, and I could immediately feel the difference, even at 1500 rpm. After getting up to temp, I easily backed her down to ~750 strong and made sure I was getting the most vacuum with the mixture screws for that rpm. (~16.5 inHg) Smooth as a top :rockon

Had to go for a ride with the bird dog and man she runs like a bat outta hell now!

Thanks again for everyone that contributed to this thread. Even though half the ideas weren't the culprit, it made me step through the process and get super familiar and comfortable with this engine... Never make the assumption that just because you cleaned a few things, the rest is fine.
 
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