Internal vaccuum leak on 1973 Camaro? - Team Camaro Tech
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post #1 of 16 (permalink) Old May 10th, 17, 01:41 AM Thread Starter
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Tobias
 
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Internal vaccuum leak on 1973 Camaro?

Hi Guys!
I am new to the Forum. I recently bought a 1973 Camaro. It was a barn find that had been sitting for ten years. It`s far from mint condition but it`s a start and I like it.
It`s got a 350 4 BBL with Edelbrock Performer Carb and intake manifold.
I got it back up and running and got to drive it for around 200 miles. Then, out of nowhere it would stop idling properly (while I left it at the emissions check and technical testing). I checked ignition Timing, replaced plugs and parts of the HEI Distributor. No Change. I did an overhaul of the carb. No Change. I unhooked all vacuum lines. No Change. Fuel filters are fine too. Adjusting the idle screws makes no difference.


Now here`s the Thing. There is no electric / manual choke attached to the Edelbrock Carb but if I hold the choke shut it will suddenly idle properly, If I release it, it will almost stall. (It will run, but it shakes and Sounds like a tractor). When pressing down the accelerator it will run somewhat fine, but at unsteady rpm. When I pull the plugs they are grayish White, pointing to a lean mixture. A meter attached to the carb will Show little, unsteady vacuum at idle, somewhat steady and higher vacuum when revving the engine.


For me, all this Points to a vacuum leak. Problem is, I can`t find anything on the outside of the engine. I am afraid I might have an internal vacuum leak. My question is: Does anyone have a clue for me what I could check BEFORE removing the intake manifold and check for cracks / broken seals??


I have some Quadrajets lying around from my 1979 Oldsmobile Custom Cruiser (403 CID). Will this carb fit on the Chevrolet Engine? If so, I might be able to exclude the Edelbrock Carb as the source of my Troubles.


Many thanks in advance!


Tobias
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post #2 of 16 (permalink) Old May 10th, 17, 06:04 AM
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Re: Internal vaccuum leak on 1973 Camaro?

Welcome, Tobias, to the Club.
Congrats on the new buy and enjoy the journey.

Not an expert here, but, does the exhaust tone sound like a aftermarket camshaft with longer duration?
OE designed camshafts have a smooth idle, generally 18-22 inches of vacuum.
Longer duration design closes the intake later in the cycle and engine at lower RPM, creates less vacuum.
Any camshaft card in glove box? Previous owner know anything?
You might have to bolt on a degree wheel and measure valve lift & duration and compare to stock camshaft specifications.

You could install the Rochester carb and compare results, since 403, it may be a little 'fat.'.

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post #3 of 16 (permalink) Old May 10th, 17, 06:36 AM
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Re: Internal vaccuum leak on 1973 Camaro?

Do you have an automatic transmission? Check the vacuum line from the intake to the vacuum modulator.

Could also be an issue with the carb. Edelbrocks are notorious for having issues. I would switch it or a Quick Fuel Carb.

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post #4 of 16 (permalink) Old May 10th, 17, 07:02 AM Thread Starter
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Re: Internal vaccuum leak on 1973 Camaro?

Thanks for the answers and hints!
I do have automatic Transmission - but I already tried unhooking the vaccuum line to it and there was no Change in the Problem.

Unfortunately the previous owner is dead and there was no documentation in the car, so I do not know a lot about its history. However I believe that the cam is stock or maybe stage 1. Nothing more. The engine had been running smoothly for several hours over the past weeks. The issue just came up now.
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post #5 of 16 (permalink) Old May 10th, 17, 08:53 AM
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Re: Internal vaccuum leak on 1973 Camaro?

What is your initial timing and total timing? Shoot for 14*-16* initial which will get you to 36* total.

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post #6 of 16 (permalink) Old May 10th, 17, 09:48 AM
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Re: Internal vaccuum leak on 1973 Camaro?

If you have a propane torch, it can be used to find vacuum leaks at places like manifold to carb. To do so, flow UNLIT, UNLIT, UNLIT propane from the torch, around carb. Also can be sued to find leanness in the carb once vacuum leaks are found/fixed, or proven no leaks.

As far as timing, you probably have an engine that used 8/10 degrees of initial timing, NOT 16. Set it to 12 maximum with an aftermarket distributor. NOW, to do the vacuum advance correctly to work with the 12 initial, and full manifold vacuum, you need to fit a stop, either home made, or Crane 99619-1, NOT expensive, to the vacuum advance, and set it to give a total IDLE timing of 22/24. This would be the combination of initial and vacuum degrees, so, 12 +10 = 22, 12 + 12 = 24. Then, once the stop is in and set, connect the vacuum advance to full manifold vacuum.

If you want a full set of instructions and pictures on how to do the above, send me an email and I will send the whole package, IT IS FREE OF CHARGE.

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You said the intake manifold is a Performer, is it one of the later ones with a functioning EGR valve in place?? I doubt it has an EGR valve.
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post #7 of 16 (permalink) Old May 10th, 17, 11:03 AM
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Re: Internal vaccuum leak on 1973 Camaro?

Just a few thoughts. Did you replace the fuel pump? May not be pumping enough fuel at idle(hole in the diaphragm). Clogged air bleeds will cause idle issues.

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post #8 of 16 (permalink) Old May 11th, 17, 10:21 AM
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Re: Internal vaccuum leak on 1973 Camaro?

If the HEI is equipped with a vacuum advance canister, be sure that the diaphragm in that unit is good. A ruptured diaphragm can create a vacuum leak in addition to depriving the engine of needed advance in high vacuum conditions. Just a thought.

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post #9 of 16 (permalink) Old May 12th, 17, 01:00 AM Thread Starter
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Re: Internal vaccuum leak on 1973 Camaro?

Ignition timing is certainly not finetuned properly - Thing is I tried turning it to almost every position and there was no change to the problem. The only way I can influence idle "Quality" is by holding the Choke shut.
The fuel pump is new - and I have a glass window filter attached AFTER it, so I can see that there is plenty of fuel coming to the carb. I also tried removing the filter as it may be clogged but that made no difference either.
My HEI does have vacuum advance but I tested it by sucking on it and it seems to fine. Also tried unhooking it and plugging the port at the carb. The engine idled at a different pace then of course, however it did not resolve the issue.

I will try Fitting the Quadrajet from my Oldsmobile on the Weekend and see if that works. If so, it would point to an issue in the carb. Also, I will get a propane torch (thanks for this advice, haven`t tried that yet) and have another look at the ignition Timing.
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post #10 of 16 (permalink) Old May 12th, 17, 06:52 AM
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Re: Internal vaccuum leak on 1973 Camaro?

Three other items to check;

1. PCV valve, make sure it is not stuck in the open position from being gunked up or a broken plunger spring. If it is, you essentially end up with a vacuum leak the size of a 3/8" diameter opening.

2. If you have power brakes, make sure the plastic check valve that plugs into the rubber grommet of the brake booster isn't cracked. I once had an intermittent vacuum leak and found the plastic check valve for the brake booster was cracked 3/4's of the way around the nipple where it met the valve body. Tension on the vacuum line from various forces in the engine bay would cause the crack to open at times and the engine would lose vacuum and almost die.

3. Check all rubber vacuum plugs for cracks and brittle rubber, I had one blow off a large vacuum nipple on the rear of a carburetor one time when the engine backfired, the engine wouldn't idle after it happened (this also creates a vacuum leak the size of a 3/8" diameter opening). It was one of the newer thin vinyl vacuum plugs rather than an old school thick rubber vacuum plug. I did manage to find it and reinstall it along with a small black nylon wire tie to secure and prevent a repeat in the future

In my experience, vacuum leaks like you describe are usually external to the engine, especially if it is intermittent. Good luck and let us know what you find?

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post #11 of 16 (permalink) Old May 12th, 17, 09:43 AM
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Re: Internal vaccuum leak on 1973 Camaro?

Serious vacuum leak. Be aware, there are quite a few different carb to manifold gaskets for different applications of various carbs. All depend on correct carb pad sizing on the manifold, and the carb used.

In the case of a Q-Jet manifold, there are at least 7 different gaskets, all will cause leaks IF the wrong one is used.

On a Q-Jet manifold, with a square flange carb, there are a few different situations that can cause problems, like the bores in the manifold too large to seal the square flange carb. Edelbrock has a fix for the more mundane interface options, their adapter plate, p/n 2732, flat steel, covers the Q-Jet bores, mates with the square flange carb, stops leaks. then, there are various adapters that are thicker, do the same thing.

On some square flange manifolds, there might not be enough metal on the sides of the carb pad to properly support the carb to manifold gasket, allowing it to 'droop', and cause a series of small air leaks, that become one large one. Same fix shaould work 2732 will seal the plate to the manifold, and carb to the plate.

There are options, but, first, find the leak, if you have it, sue the propane, it works well.
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post #12 of 16 (permalink) Old May 12th, 17, 03:42 PM
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Re: Internal vaccuum leak on 1973 Camaro?

Kind of breezed through your issue - is the engine equipped with an EGR valve?? If so, be sure it's not stuck open, or you will have this issue as well. FWIW
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post #13 of 16 (permalink) Old May 17th, 17, 02:16 AM Thread Starter
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Re: Internal vaccuum leak on 1973 Camaro?

Thanks so much for all your hints!

Meanwhile I fit a (recently bought, overhauled Autoline) Quadrajet from my Oldsmobile on the Camaro Engine. It fits, even if you need a little creativity here and there. And now guess what; It idles much smoother now. Not as good as it used to run with the Edelbrock before it went bad, but then again I fit the carb with some base gasket I had lying around. That is probably not 100% tight. Also, some of the vacuum plugs may be worn etc. Ignition Timing is set just by vaccuum gauge. It should read between 18 -23, mine reads about 17. It's a start and it is much better than before. Now it needs finetuning.

So it seems I had an internal vaccuum leak - but in the carb. Again: It ran and idled fine and then suddenly went bad. I tried overhauling it with the appropriate kit and it did not resolve the issue. Can a Edelbrock carb crack or break internally? I saw nothing when overhauling it....

Another Question: With no emissions systems installed and for a non-racing setup the Distributor vaccuum advance must be connected to manifold vacuum, right? (instead of ported)

p.s.: I have no EGR or PCV Valve installed.
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post #14 of 16 (permalink) Old May 17th, 17, 08:38 AM
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Re: Internal vaccuum leak on 1973 Camaro?

Most factory is ported for smog issues but manifold would be preferred. Sounds like you solved you vacuum leak but I thought I would toss this out.

I have seen a vacuum leak that was internal and hard to trouble shoot. It was on my buddy's DZ302, he installed one of the lift plates with slightly longer than factory bolts and they kept the intake from seating properly but not enough to be detected visually.

ps. install a PCV valve, you want the crankcase ventilated and burning off the crankcase fumes is a good way to do it...

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post #15 of 16 (permalink) Old May 17th, 17, 10:44 AM
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Re: Internal vaccuum leak on 1973 Camaro?

I see a couple of issues already with what you have given in your last post.

First, HOW MANY CRANKSHAFT DEGREES OF TIMING DOES THE VACUUM ADVANCE DELIVER? If over 10, you will need to make a stop to restrict the degrees down to 10 maximum.

You state the timing is set by "vacuum gauge"?????? THAT IS NOT THE PROPER WAY TO DO IT. You need to use a timing light, with any dial back feature left OFF.

If you want to do the vacuum advance correctly for full manifold vacuum, send an email and a whole set of instructions and pictures will be sent to you, absolutely free. They work with either a stock non-adjustable, or fully adjustable vacuum advances, points or HEI.

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