# High Idle RPM for 67 GTO 400 with Restored Quadrajet



## rickmpontiac (Jul 31, 2015)

I recently replaced the intake manifold and carburetor on my 67 GTO to get back to the original configuration. Removed an Edelbrock #1407 and Edelbrock Performer manifold and installed the correct cast iron manifold and a 7027262 Quadrajet. The carburetor was fully restored. Now I can't get my engine to idle below 1,000 RPM. The curb idle screw is backed all the way out, and the fast idle cam is not engaged. I had a suggestion on another forum to plug the hot idle compensator. Checked that and it was already plugged. I've removed the base plate of the carburetor and all the throttle blades appear to be centered in their respective holes in the base plate. I can reduce the idle by removing the vacuum advance hose as you would expect. I noticed my vacuum advance is increasing the timing by 24-25 degrees, which is higher than it should be. 

All the vacuum hoses are new, and there doesn't appear to be a vacuum leak. I've tuned the carburetor using a vacuum gauge, and have 16-18" at 800 RPM w/o vacuum advance connected.

Any thoughts or suggestions?

Thanks in advance.


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## PontiacJim (Dec 29, 2012)

rickmpontiac said:


> I recently replaced the intake manifold and carburetor on my 67 GTO to get back to the original configuration. Removed an Edelbrock #1407 and Edelbrock Performer manifold and installed the correct cast iron manifold and a 7027262 Quadrajet. The carburetor was fully restored. Now I can't get my engine to idle below 1,000 RPM. The curb idle screw is backed all the way out, and the fast idle cam is not engaged. I had a suggestion on another forum to plug the hot idle compensator. Checked that and it was already plugged. I've removed the base plate of the carburetor and all the throttle blades appear to be centered in their respective holes in the base plate. I can reduce the idle by removing the vacuum advance hose as you would expect. I noticed my vacuum advance is increasing the timing by 24-25 degrees, which is higher than it should be.
> 
> All the vacuum hoses are new, and there doesn't appear to be a vacuum leak. I've tuned the carburetor using a vacuum gauge, and have 16-18" at 800 RPM w/o vacuum advance connected.
> 
> ...


Saw your post over on PY. 

Vacuum leak fixed under the carb? Does the vacuum gauge needle remain steady - no fluctuations?

Is your engine/cam stock?

What is your initial timing with the vacuum advance disconnected? The vacuum will cause your timing to advance if it is connected directly to engine vacuum. Your initial is most likely 5-6 degrees? Add to it 20 degrees from the vacuum advance and you have your 24-25 degrees total at idle as you have. Your vacuum advance is "direct" engine vacuum, versus a ported vacuum source - so the increase in timing at idle is normal.

With the vacuum advance disconnected, can you drop/adjust your idle speed using the idle speed adjustment on the carb?

No leaks between the intake/heads - bad intake gasket? Bolts torqued down after changing the manifold?

Cliff Ruggles gave you good advice on the distributor as it could be a distributor/timing issue.

How did you adjust your idle mixture screws? How many turns out from "bottom".

Is the air cleaner element new? Have you tried it without the filter? Sometimes they can look good, but not flow well and this can cause a "rich" condition.

PCV system working? Try plugging the hose to test - ie to eliminate a possible vacuum problem.

Valve cover breather functioning - not plugged with crud?

Have you checked the power brake check valve found at the booster - if you have power brakes. If the check valve goes bad, or the brake booster diaphragm is leaking, it will act like a vacuum leak. Again, if you have this, pull it and plug it to check for a vacuum leak or non-operating check valve.

Check a few of these things above and report back to us. You may have to narrow the problem down by knowing what it is not. Test 1 thing at a time and if it does nothing, return the adjustment back to where it was so it does not interfere with another test.


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## BearGFR (Aug 25, 2008)

rickmpontiac said:


> I recently replaced the intake manifold and carburetor on my 67 GTO to get back to the original configuration. Removed an Edelbrock #1407 and Edelbrock Performer manifold and installed the correct cast iron manifold and a 7027262 Quadrajet. The carburetor was fully restored. Now I can't get my engine to idle below 1,000 RPM. The curb idle screw is backed all the way out, and the fast idle cam is not engaged.


Ok, to make sure I understand. Everything is the same except for the intake and carb, correct? That will rule out things like a leaking power brake booster, for instance. If the idle is too high after the change and was fine beforehand, it means that the engine is able to get both extra air and fuel from -somewhere-, and it's most likely related one of the things you changed.

You mentioned it idling lower with the vacuum can disconnected. That makes me think it's connected to a straight manifold source and not a ported source. Try moving it to a ported source.

Disconnect and cap off/plug every vacuum connection you can find. What happens with that? Reconnect them one at a time until you find one that makes it different.

Where are the idle screws set on the QJet? Lightly seat them then back them out no more than 2 full turns, maybe 1 1/2. Does it run ok with that setting? If it runs rough/lean there (use a vacuum gauge) then you've got a vacuum leak somewhere. Check all the mating surfaces (carb base, where the intake meets the heads) over with a can of spray carb cleaner or starting fluid. If you find a spot where spring there makes it clean up and idle better, you've found a leak. 

If you haven't found any leaks and it's idling clean and good with the idle screws no more than 2 turns out, the curb idle RPM screw is backed all the way out, you're certain that the throttle plates are completely shut at that point, and it still won't idle down (or will still run at all with all vacuum disconnected and capped) try capping off all the vacuum fittings and slowly closing off the idle screws. You should reach a point where it won't run at all. If it still runs with them seated, then that carb is still passing both air and fuel when it shouldn't be. Suspect a cracked body, warped base plate, or some other problem with the carb itself. 

Bear


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## Lemans guy (Oct 14, 2014)

You don’t want vac advance to increase timing by 24 or 25 degrees, ever....10 more is all you want from vac....disconnect and plug like the guys said and see what happens


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## Lemans guy (Oct 14, 2014)

That much timing at idle will increase your idle RPM’s. But if your cam is not stock and hotter expect a higher idle RPM than from factory original.....as that lope gives more reversion and dilutes the air/fuel mixture.

So you still may need a higher idle RPM than factory specs for that year car..


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## AZTempest (Jun 11, 2019)

All good advice. I recall a gentleman on the forums here having a problem with his engine acting like a vacuum leak after some work on the engine. Turns out it was the intake hitting the valley pan not allowing the intake a good seal. Just throwing that out there.


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## rickmpontiac (Jul 31, 2015)

Thanks for all the good advice and lists of things to check. I'm starting to think there are multiple issues in the mix. Someone asked about my cam - I have a cam manufactured by PAW years ago that's very similar to an Edelbrock Performer cam. Not sure how they were not a factor prior to switching out the intake and carburetor. I checked the power brake booster using the method that most say is a good test and got marginal results, but I tried using a handheld vacuum pump to pull a vacuum in the booster and get nothing. I can reduce the idle RPM (at operating temperature, no fast idle engaged), down to the desired level of about 600 RPM, by removing the power brake booster vacuum hose and plugging it. My vacuum advance does add 24-25 degrees bringing my initial timing of (6 degrees without vacuum advance connected) to about 31 degrees total. I can't be certain it has the correct vacuum advance on it right now, so I thought that would be a fairly inexpensive starting point for parts, but can't find an ACDelco VA for the car. I prefer to avoid the store brands. I would like to get some validation from you guys that I should be able to pull a vacuum in the power brake booster using a handheld vacuum pump before I order parts. Appreciate all of you providing feedback!


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## 67ventwindow (Mar 3, 2020)

Do you have any brake boost after the engine is shut off? I had many sketchy cars thru the years that I depended on that vacuum in the booster to get it woowed down after the engine stopped due to bad gas gauge, fried my condensor, HEI module layed an egg., Battery arced out on my inner fender.(Dont play dukes of hazard without a battery hold down). etc. Not saying yours is sketchy I tend to drive the wheels off my daily drivers. Had a hydraboost on a GMC. Engine dies you better stand on your brakes because its not stopping on this side of town. I dont have much knowledge on GTOs but I have not read to anything about booster issues. The mercury forum thats another story, 

Good luck to you. The guys here have help me with out knowing. I use some thier principles on the smorgasboard of cars I have here.


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## Mine'sa66 (Oct 30, 2019)

rickmpontiac said:


> Thanks for all the good advice and lists of things to check. I'm starting to think there are multiple issues in the mix. Someone asked about my cam - I have a cam manufactured by PAW years ago that's very similar to an Edelbrock Performer cam. Not sure how they were not a factor prior to switching out the intake and carburetor. I checked the power brake booster using the method that most say is a good test and got marginal results, but I tried using a handheld vacuum pump to pull a vacuum in the booster and get nothing. I can reduce the idle RPM (at operating temperature, no fast idle engaged), down to the desired level of about 600 RPM, by removing the power brake booster vacuum hose and plugging it. My vacuum advance does add 24-25 degrees bringing my initial timing of (6 degrees without vacuum advance connected) to about 31 degrees total. I can't be certain it has the correct vacuum advance on it right now, so I thought that would be a fairly inexpensive starting point for parts, but can't find an ACDelco VA for the car. I prefer to avoid the store brands. I would like to get some validation from you guys that I should be able to pull a vacuum in the power brake booster using a handheld vacuum pump before I order parts. Appreciate all of you providing feedback!


Your brake booster should hold vacuum. If you try to pull a vacuum on it with a Mighty Mite or something and it won't, you're leaking. Be sure though that it's not just the check valve that's leaking.


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## rickmpontiac (Jul 31, 2015)

Mine'sa66 said:


> Your brake booster should hold vacuum. If you try to pull a vacuum on it with a Mighty Mite or something and it won't, you're leaking. Be sure though that it's not just the check valve that's leaking.


Thanks, I've since determined that I'm losing vacuum around the mounting base of the master cylinder, so it's time to order a new one brake booster. I replaced the check valve a while back and it still works.


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## PontiacJim (Dec 29, 2012)

rickmpontiac said:


> Thanks, I've since determined that I'm losing vacuum around the mounting base of the master cylinder, so it's time to order a new one brake booster. I replaced the check valve a while back and it still works.


Your welcome. I may be wrong, but wasn't that on my list of things to check in Post #2? I am the greatest.


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## rickmpontiac (Jul 31, 2015)

PontiacJim said:


> Your welcome. I may be wrong, but wasn't that on my list of things to check in Post #2? I am the greatest.


Thanks Jim, yes I was working through the different lists of things to check provided by everyone, but I was skeptical about this since the car idled fine with the Edelbrock carburetor. I'll be back again to challenge you with another issue


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## Lemans guy (Oct 14, 2014)

Good call PJ, those vacumn leaks are a pain and with mess up your idle, there are various methods to find them I use a smoke machine it is easier, but sounds like rick is on it!


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## PontiacJim (Dec 29, 2012)

Lemans guy said:


> Good call PJ, those vacumn leaks are a pain and with mess up your idle, there are various methods to find them I use a smoke machine it is easier, but sounds like rick is on it!


Yep, I just recalled a couple past members having this issue and searched all over and checked/replaced a bunch of stuff and it turned out to be in one instance the check valve going into the booster, and another was the rubber seal/grommet the check valve fits into, and the other instance a leaking vacuum booster as the rubber diaphragm had developed a hole in it.

I have never really had any vacuum issues and those I did were quite obvious, like a bad carb gasket or one that was a mismatch when using an adapter. Most guys won't have a "smoke machine" and it has been said you can use WD-40, Starting Fluid, or even the Propane Gas from one of the small hand held propane torches. Starting fluid in/on a running engine kinda scares me a bit and my lucj is that it will catch something on fire. LOL. WD-40 might be safest, but it is kinda heavy being a liquid and don't know if it would work so well when distance from the leak to the carb is a long length away - such as the power brake booster. So my money would be on the Propane torch.


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## Lemans guy (Oct 14, 2014)

Those will all work some better than others,....just have a fire extinguisher nearby.

if you suspect a small leak at a base gasket you can dab it with some RTV sealant, let it set up dry and then start the engine and see if it improves, if so you may have found the leak..


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## rickmpontiac (Jul 31, 2015)

I was planning to use propane as others had suggested to identify a leak, but couldn't find my torch valve. It's one of those things I use about every 20+ years. In the meantime I decided to go old school and use a piece of half inch hose as a ******* stethoscope to listen around the suspect vacuum connections and there was no doubt about losing vacuum around the base of the master cylinder. A new brake booster is on the way along with a new master cylinder. Couldn't see not replacing the latter with everything already removed from the car. Thanks again everyone!


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## Jd70 (Jul 4, 2019)

rickmpontiac said:


> I was planning to use propane as others had suggested to identify a leak, but couldn't find my torch valve. It's one of those things I use about every 20+ years. In the meantime I decided to go old school and use a piece of half inch hose as a ******* stethoscope to listen around the suspect vacuum connections and there was no doubt about losing vacuum around the base of the master cylinder. A new brake booster is on the way along with a new master cylinder. Couldn't see not replacing the latter with everything already removed from the car. Thanks again everyone!


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## Jd70 (Jul 4, 2019)

I vote vacuum advance incorrectly connected to manifold vacuum. A leaking booster would cause you to turn UP idle speed to keep it running. Unless you have a really screwed up carb rebuild thats overly rich to make up for holey booster, without a cranked up idle screw. When you unhooked dist. vacuum line the over advanced timing was returned and you had a slower idle, thats the clue. 

JD


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