# Vacuum leak?



## mbergin (Jun 19, 2012)

Hi all. New to the forum and looking for some help. 
I have a 69 GTO 4 speed with a 428 (non original). Bought the car with 100 miles on a rebuilt motor. The motor shakes a lot when idling and I contacted the rebuilder who said he installed a comp cams extreme cam ( no other info) but I'm told the motor should still run smooth. I had the carb rebuilt but the motor still shakes at an idle as it did with the first carb. When I put my hand over the primary butterfly, the idle comes up and the motor smooths out. I did some research and noted that the 4 speeds did not come with the temperature vacuum switch so I pulled it and plugged the manifold opening with a NOS plug. I hit the whole manifold with carb cleaner with the engine idling, noted no change and still can't seem to find the vacuum leak. Two questions: 
First, does anyone have any idea how I can find this leak? Is it a vacuum leak? 
Second, on the 69 GTO 4 speed, where does the vacuum advance line get attached? I have it attached to the ported line on the carb. 
Would really appreciate any help. Thanks to all. 




Sent from my Autoguide iPad app


----------



## 05GTO (Oct 6, 2004)

Welcome to the forum,

Unfortunately, what you are describing are the characteristics of the COMP Xtreme Energy Cam, I believe they run a 110 degree LSA and the stock GTOs of the 60s and 70s used cams with 113-116 LSA. The rough idle and loss of vacuum are part of the swap off for the additional low end power.
See if the engine builder can shed some light on which cam he used and the specs,


----------



## mbergin (Jun 19, 2012)

Thanks so much for the reply 05. Not to sound too much like a moron but what does LSA stand for? I had a buddy that put a killer cam in a 455 and it ran smooth as silk. I put a vacuum gauge on the manifold and it was jumping quite a bit between 13 and 15. So it's normal that the car runs smooth when the butterfly is covered? I really appreciate the help. Thanks so much!


Sent from my Autoguide iPad app


----------



## geeteeohguy (Feb 2, 2008)

If it speeds up when you choke the carb it is too lean in the idle circuit. This could be an air leak or the carb calibration itself. Have you tried turning the mixture screws counter-clockwise to richen it up??


----------



## mbergin (Jun 19, 2012)

I tried to adjust the mixture screws. Should have included that info in my original post. I can bury or back out the right screw with very little change. If I try to turn the left screw (driver side) in, the car dies. I installed the other carb, same issue. I'm stumped..


Sent from my Autoguide iPad app


----------



## mbergin (Jun 19, 2012)

Backing the mixture screws out makes the car stumble worse..


Sent from my Autoguide iPad app


----------



## 05GTO (Oct 6, 2004)

mbergin said:


> Thanks so much for the reply 05. Not to sound too much like a moron but what does LSA stand for? I had a buddy that put a killer cam in a 455 and it ran smooth as silk. I put a vacuum gauge on the manifold and it was jumping quite a bit between 13 and 15. So it's normal that the car runs smooth when the butterfly is covered? I really appreciate the help. Thanks so much!
> 
> 
> Sent from my Autoguide iPad app



Lobe-Separation Angle (LSA)
Indicates the angle, in camshaft degrees, between the maximum lift points on the intake lobe and the exhaust lobe. Lobe separation is usually calculated by adding the intake centerline and the exhaust centerline, then dividing by two. For example, a cam with a 106* intake centerline and a 114* exhaust centerline has an LSA of 110* [(106+114)/2=110]

Narrower LSA
Increased overlap
Increased low-rpm torque
Narrower powerband
Reduced idle quality
Increased cranking compression
Decreased piston-to-valve clearance

Wider LSA
Reduced overlap
Improved top-end power
Wider powerband
Improved idle quality
Reduced cranking compression
Increased piston-to-valve clearance


----------



## pontiac (Mar 6, 2011)

what intake? if one screw affects the engine, and the other does not on two different carbs, then vacuum leaks are not the issue.


----------



## BearGFR (Aug 25, 2008)

Sounds like it's way lean since it cleans up when you restrict the air flow. Idle screws not having any (or much) effect can be an indication that your idle circuit in the carb is not passing enough fuel/air to keep the car running, so you have to adjust the idle speed screw "up" quite a bit --- doing this begins to open the throttle plates enough so that the transition circuits in the carb start to operate (translation: the engine is running off the 'main' system and not off the idle system, so the idle mixture screws won't really do anything). 
First order of business: look "everywhere" for an open vacuum port. You might have a vacuum line that's leaking (cracked), leaking power brake booster, or perhaps a port on the back side of the carb that has been left disconnected/open. PCV valve could be stuck open... if it's an automatic, the vacuum modulator on the trans could be ruptured/leaking. You could always disconnect "everything" (power brake booster, pcv valve, trans modulator, vacuum advance, etc.) and plug the ports. Put your detective hat on. The fact that it cleans up when you restrict the air flow is telling you that you're getting way too much air into the engine - from somewhere. It could even be a cracked intake manifold. Start with the most likely and keep hunting.

Bear


----------



## mbergin (Jun 19, 2012)

Thanks so much Bear. I've been on this for two months. Since the car is a 4 speed, I removed the tvs and plugged the manifold. I've been at the car with carb cleaner trying to locate the leak. I'll plug everything and see if that works, then one by one reconnect until I find the source. much appreciate the help. I'll let you know how I make out. Mike


Sent from my Autoguide iPad app


----------



## Rukee (Feb 8, 2007)

Starting fluid may locate the leak better then carb cleaner. Keep a fire extinguisher handy.


----------



## mbergin (Jun 19, 2012)

Hi Bear. I plugged all the vacuum lines and now when I put my hand over the carb she dies. Progress. Problem is I still can't adjust the mixture screws since I can bury them or back them out with no difference. Motor is still shaky but I'm thinking that may be the cam. Any more thoughts?


Sent from my Autoguide iPad app


----------



## BearGFR (Aug 25, 2008)

Does indeed sound like progress. Try this: adjust the idle speed screw up so the motor runs at about 2000 rpm. use your hand to slowly -begin- to restrict the air going into the carburetor (don't shut off the air completely) a little bit at a time. When you first start to restrict the air flow, does the engine rpm increase? decrease? stay the same?
If it changes, by how much? (you'll need an accurate tach for all this). If it speeds up by a lot (more than 50-75 rpm) then the fuel mixture is way too lean. Of course if you continue to restrict the air flow the engine is eventually going to slow down and die --- what you're interested in here is finding out what happens when you reduce the air supply by just a little bit.

Don't fret too much yet about the idle mixture screws being ineffective, that's just an indication of what I said earlier --- the engine is actually running mostly off the mains and not the idle circuit. 


What kind of carb is this again?

Bear


----------



## mbergin (Jun 19, 2012)

Thanks Bear. Its a 69 Rochester quad, just rebuilt. 


Sent from my Autoguide iPad app


----------



## mbergin (Jun 19, 2012)

I'll give it a try with the slow adjustment but I was able to restrict all of the air (mixture screws buried) and there was no change. 


Sent from my Autoguide iPad app


----------



## geeteeohguy (Feb 2, 2008)

I'd LOVE to take a look at this thing. Frustrating. How well does it run going down the road? Also, what are your vacuum gauge readings? Depending on camshaft and engine condition, a vacuum gauge can speak volumes. If the thing is stll running when you restrict ALL of the air, that would be a miracle. Pretty much impossible to do that. The engine needs some air going thru the carb to run!! What's your location??


----------



## Rukee (Feb 8, 2007)

If the cam is so big you need to crank the idle screw in to keep it running, that will cause the mixture screws to not make any diffrence as well.


----------



## mbergin (Jun 19, 2012)

Thanks Rukee for the insight and the tip to use starting fluid. GTO guy- I'm in derry nh. The motor runs with the mixture screws all the way in. 
Bear-I called the previous owner who ensured that the car ran smooth for him and the cam was not so big that it would cause a rough idle. I followed your directions brought the idle up to 2K and slowly turned the mixture screws in. No change. The vacuum gauge bounces between 13 and 15. The car runs down the road great but not with the power that a 428 with 360hp and 472 ft/lbs of torque should. At an idle, the engine stumbles and jumps. I called the carb rebuilder who told me to turn the idle as low as it can go and try to adjust the mixture. I cant get the car to run below 750 rpms and the adjustment showed no change. I'm at a loss. 


Sent from my Autoguide iPad app


----------



## mbergin (Jun 19, 2012)

Gto guy, the engine only has 200 miles on it, professionally rebuilt from a reputable shop in Mass. 


Sent from my Autoguide iPad app


----------



## BearGFR (Aug 25, 2008)

mbergin said:


> Thanks Rukee for the insight and the tip to use starting fluid. GTO guy- I'm in derry nh. The motor runs with the mixture screws all the way in.
> Bear-I called the previous owner who ensured that the car ran smooth for him and the cam was not so big that it would cause a rough idle. I followed your directions brought the idle up to 2K and slowly turned the mixture screws in. No change. The vacuum gauge bounces between 13 and 15. The car runs down the road great but not with the power that a 428 with 360hp and 472 ft/lbs of torque should. At an idle, the engine stumbles and jumps. I called the carb rebuilder who told me to turn the idle as low as it can go and try to adjust the mixture. I cant get the car to run below 750 rpms and the adjustment showed no change. I'm at a loss.


I guess I wasn't clear enough - sorry about that. Run it up to 2000 with the idle speed screw then use your hand ("tip" it in over the top of the carburetor) to restrict air flow. What we want to know is how does the engine react when you start to take its air away that way. Idle screws won't (and shouldn't) do anything at 2000 rpm --- by then it's running completely on the main metering circuit.

Bear


----------



## BearGFR (Aug 25, 2008)

Any chance you could shoot a short video of the motor "shaking" and put it out on youtube or something? It'd help to be able to see what we're dealing with here.

Bear


----------



## mbergin (Jun 19, 2012)

Hi Bear. I raised the idle up to 2K and slowly restricted the air to the primaries with my hand. No change until I almost blocked off the complete butterfly, at which point it wanted to stall. Here is the video of the motor shaking;





Let me know what you think
And thank you so much 





Sent from my Autoguide iPad app


----------



## mbergin (Jun 19, 2012)

Here's another angle. The vacuum lines are disconnected and plugged. 






Sent from my Autoguide iPad app


----------



## BearGFR (Aug 25, 2008)

I see what you mean about it moving around, but honestly it doesn't _sound_ bad at all. Sounds reasonably healthy in fact. How is it driving? Does it try to die on you when you stop at a light? I tried to compare to some of my videos of mine, but I don't really have any where I stand still with the camera long enough to get a good comparison . I'm going to be working on my carb some tonight though, so I'll pay attention.

Bear


----------



## mbergin (Jun 19, 2012)

Thanks Bear. I can't bring the idle below 750 or she'll stall. That's really not a big deal since the idle spec is 1000. She drive ok but seems like she lacks power and the shifter shakes like a harley handlebar. Let me know what you find out tonight. 


Sent from my Autoguide iPad app


----------



## mbergin (Jun 19, 2012)

So based on the videos, should I not be concerned about the shaking and the inability to adjust the carb?


Sent from my Autoguide iPad app


----------



## BearGFR (Aug 25, 2008)

Well, I paid attention to mine (didn't take any video yet) and it doesn't move around as much as yours does, and I've got a moderately rowdy solid roller cam, so perhaps yours is a little 'much'. Have you made sure all your wires are good and all cylinders are firing?

As far as the idle screw adjustment, that's a completely different issue - get yourself a copy of that book I pointed you to (available through all the usual sources like Barnes & Noble, Amazon, etc.) - pages 90-93 talk about how to add idle bypass air.

Basically, the problem is caused by the combination of a large displacement engine and a more aggressive than stock cam. The stock Q-jet idle system on "most" carbs can't meet such an engine's air needs at idle via the idle circuit alone, so in order to make the engine idle one has to get more air into it "somehow". There are 3 ways to do this: 
1) turn up the idle speed screw
2) drill 1 small hole in each of the primary throttle plates
3) "enable" idle bypass air

1) works, but has the side effect of rendering the idle mixture screws useless
2) can also work, but if you get the holes too large then you get to start over with a new set of plates - and this also can affect the effectiveness of the idle screws.
3) is the preferred method. Many "Performance" QJets, such as the 455 SD unit for example, come from the factory with this system functioning. Others have the passageways in place for it but they're just not open. Some don't have provisions for it at all (on these, you either have to use a different carb body and throttle body or drill holes in the throttle plates)

Bear


----------



## mbergin (Jun 19, 2012)

Thanks Bear. You gave me quite a bit to think about. I've ordered the book and will take it from there. I appreciate your help and will let you know how I make out.
Mike


Sent from my Autoguide iPad app


----------



## geeteeohguy (Feb 2, 2008)

My seat of the pants perception: Running too LEAN. It's Running on all 8, no dead holes, cam is a small part of the roughness factor, but it is running _LEAN_. I can hear an air leak on the vids, too. Do you have the right intake gaskets installed? What about baseplate gaskets.


----------



## mbergin (Jun 19, 2012)

Thanks geeteeohguy. Maybe the air leak you hear is from the carb? It goes away with the air cleaner on and the carb is sucking a lot of air from the primaries. Base gasket is a felpro for a 69 manifold with a qjet. Not sure about the intake gaskets since I did not assemble the motor. I hit everything with carb cleaner including the manifold/head mating sufaces and noted no change. Next step may be pulling the intake to see what I got. 


Sent from my Autoguide iPad app


----------



## chuckha62 (Apr 5, 2010)

Quick thought... Make sure the carb base gasket is installed right side up. On my '67, there is a passage (the heat riser crossover) that opens the base of the carb up to the atmosphere if the gasket is installed upside down. It was exactly the same on my '69 'Vette. It may or may not be the same on your car.

Just a thought...

Chuck


----------



## geeteeohguy (Feb 2, 2008)

Chuck, also, on '67 only, make _sure_ you have the stainless seal plate along with the base gasket that seals the exhaust gas off from the carb. They tend to go missing, and cause vapor lock issues!


----------



## geeteeohguy (Feb 2, 2008)

Before you pull the intake, gin, mist a little carb cleaner down the carb while it's idling. If it smooths out and speeds up, you'll KNOW it's too lean.


----------



## chuckha62 (Apr 5, 2010)

geeteeohguy said:


> Chuck, also, on '67 only, make _sure_ you have the stainless seal plate along with the base gasket that seals the exhaust gas off from the carb. They tend to go missing, and cause vapor lock issues!


You're right! The stainless also serves as a nice insulator/heat deflector to aid in the fuel boiling issue... Mine was missing.


----------



## mbergin (Jun 19, 2012)

So I timed the motor (thanks Lars!), changed the plugs, checked the distributor for correct installation and tried a different carb and the motor still shakes. Connected the vacuum advance to a manifold port..No luck. Hand over the primaries still results in a idle increase and engine smoothing out. Gotta be a vacuum leak but I plugged them all and hit the intake/head mating surfaces with propane and carb cleaner with no change. I'm beat. Car is going to a speed shop for diagnosis and repair. Thanks to all for your help. Just to make myself feel better I washed and waxed her. Pics attached.


----------



## KingJacobo (Jul 6, 2011)

I feel your pain. I too have a vacuum leak after I replaced my leaky Carter AFB with a Holley. Runs good, but a bit of stumble 0-7 mph and 10 mpg on an otherwise stock car..


----------



## BearGFR (Aug 25, 2008)

Nice looking!... it's the right color too 

Well, at least now you've eliminated quite a number of things that it isn't. Persevere - you'll get there.

Bear


----------



## mbergin (Jun 19, 2012)

Thanks King and Bear. Just found out the shop I want to take it to is booked for 8 weeks. Hmmmmm what else can I try? Remove the intake and replace the gaskets?


----------



## Rukee (Feb 8, 2007)

try spraying starting fluid instead of carb clean, you may get better results. I've never had luck with carb cleaner. Be sure to not spray it too heavy and it's a good idea to have a fire extinguisher hand too 
Do you have power breaks? If so block that port off too to test if the booster is bad/leaking.


----------

