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Troubleshooting Diagnosing problems done here.

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post #1 of 5 (permalink) Old Aug 9th, 00, 06:20 PM Thread Starter
Join Date: Jun 1999
Posts: 93

O.K. heres the deal. 355, performer rpm manifold, Edel. 750 carb(new), mild cam, HEI(new), etc. The engine has less than 1000 miles on it.

Since it was built it just doesnt run right. Initial timing around 18 deg. (never checked total advance) cant go any lower. Put two different vacuum gagues on it and both flutter rapidly between 3 and 7 in. at idle and as engine rpm increases(might go up actually). Turn both mixture screws in all the way... still runs. Tried to cover carb opening as much as I could...still runs.

The top half of the engine was apart 2 weeks ago since it was out of the car. So the manifold gaskets were replaced. Other than carbon buildup on the plugs everything looked fine.

I have checked for vacuum leaks on top of the manifold, nothing. Tomorrow I am going to see if its leaking into the valley.

Has anybody had problems with certain manifold gaskets? Or maybe the mainfold and heads are at a slighty different angle.

What else should I check? It runs strong, just rich and rough. And I need to pass emissions in a month. I believe I have narrowed it down to a vacuum leak. But I was wondering if anyone could offer any past experiences or anything similar.

Thanks for your help. Sean.

BTW My vacuum advance is hooked up to manifold vacuum, what little I have. So I'm in the clear when the wrath of Ignition man comes. LOL

[This message has been edited by SAyers69 (edited 08-09-2000).]
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post #2 of 5 (permalink) Old Aug 10th, 00, 04:29 AM
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Join Date: Jan 2000
Location: Wilton, Ca. USA
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With your vacuum that low your metering rod springs are probably allowing the rod to raise to the power step making the engine way rich at idle. In addition to a big vacuum leak, I would check the cam timing and valve adjustment. Are the heads in good shape; freshened with the engine?

You need to fix the low vacuum which will clear up the rich idle. My guess is the cam timing is off which is why you need so much initial timing. The vacuum is so low that the metering rods in the carb are up in the power mode making your idle mixture way rich. Way too tight on the valve adjustment would act similarly.


[This message has been edited by Camcojb (edited 08-10-2000).]
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post #3 of 5 (permalink) Old Aug 10th, 00, 05:46 AM
Join Date: Apr 2000
Posts: 246

All the bells and whistles are in place for your engine to idle at a really nice 650/700 rpm in gear.

From the sound of it, one of a few things may be the culprit.

Either serious air leak where you suspect, underside of intake, at ports, but this would give a smoke cloud out the exhaust when the engine runs. If this is a small block, there are over 30 different port sizings for intake manifold gaskets, and it is easy to get a gasket that doesn't cover part of the intake to head interface. I've done it a few times when I had no time and didn't really look at the ports/gasket set sent me. Try spraying some WD40 down the PCV valve hole on the valve cover abd see if you can smell it burned out the exhaust, that'd show the leaks inside the valley, if any.

There is also a chance the intake manifold to carb interface is leaking as well, even if the correct gasket is in place there. I have found some Edlebrock stuff to need the same plate the Performer Q-jet base intake needs to use a square flange carb on it. Edelbrock part 2732. It MAYBE needs one of these, do check the gasket for imprinted pattern and any non-overlap intake to carb faces.

Also, try taking your thumb and closing off the PCV valve with the engine running, if it is a problem, the idle vacuum wilol come back up to 15/18 inches and gthe engine will idle down to a nice, normal speed. If this is the problem, there are different weighted valves available, ask for more info here. There is a very slight chance the air volume through the PCV hose is too much, if so, the passage can be ristricted with a slug, made from the unthreaded section cut from a bolt, with an .050 hole drilled through it, then installed in the carb end of the hose.

More likely, cam is retarded a tooth or the valves/lifters are too tight. If the engine doesn't miss, and idles fairly steady, I/d go for the cam being off. One thing that will kinda sorta help the idle with retarded cam is MORE timing, lots more timing, and it won't hurt starting as one would suspect and see if it were way too far advanced. If the thing will still start OK with upwards of 25 drgrees BTDC, I'd go the cam, definately.

Retarded cam will also give very poor idle & low/midrange power. but fair top end power, not great but fair.

After confirming all other vacuum related issues are in fact OK, I think I'd last ditch effort recheck the valves, engine running method is fine, you know how, back rocker off, tighten 1/4 turn past noise going away (hydraulic cam), etc. Then, if it still did the stuff its doing now, go for the cam.

Please post what you find and we'll go from there.

[This message has been edited by IgnitionMan (edited 08-10-2000).]
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post #4 of 5 (permalink) Old Aug 10th, 00, 09:46 AM Thread Starter
Join Date: Jun 1999
Posts: 93

Well I just checked for my valley vacuum leak using several methods, lastly plugging all holes and checking vacuum at the oil dipstick. Nothing, but I did look at the front of the head manifold mating surface and it seem that the gap get bigger towards the bottom of the head my maybe .015" is that normal? If I pull the intake will the gaskets tell me anything if there is a leak?

Now, on the cam when I had it all apart the marks on the crank and cam were lined up as was the timing mark on the damper, and when I was adjusting my valves (for the fourth time)turning the motor around both valves on #1 seemed to be closed.

Seeing as how I dont tear engines apart that often I cant remember how the cam is assured of being lined up on the cam gear other than the bolts, I am sure we degreed the cam, but may have screwed up.

I am beginning to think this could be the problem but hate taking off the timing cover with the engine in the car.

Is there anyway of telling for sure that the cam could be off without tearing it apart?

Thanx, Sean

[This message has been edited by SAyers69 (edited 08-10-2000).]
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post #5 of 5 (permalink) Old Aug 10th, 00, 10:14 AM
Join Date: Apr 2000
Posts: 246

On the gaskets, you would see a smaller amount of crush from manifold to head twords the valley end of the gasket than from the valve cover end, and both the faces of the gasket to the head and manifold would be wet with oil instead of dry.

There really isn't a sure fire way to tell if the cam is in correctly if the cam is a hydraulic grind, as the lifters will collapse when the engine comes to a stop. It is then impossible to read the pushrods for height. One thing you might do if the cam is a symmetrical grind (same profiles both lobes), intake to exhaust, remove both rocker arms for the no. 1 cylinder and rotate the engine to TDC on overlap, both valves still open, add a straight edge on the tops of the pushrods. If the straight edge is parallel with the valve cover rail, the cam is in properly or darn close, if the exhaust pushrod is significantly higher than the intake, the cam is retarded. This is not the best method, and you should determine if the cam has the same lift and duration for this to work right.

Let us know what you find with it.

[This message has been edited by IgnitionMan (edited 08-10-2000).]
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