Erratic Vacuum Test
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Erratic Vacuum Test
I own a 1976 Ford F with a 390 motor. Recently I just replaced the intake manifold ,timing chain, and replaced the push rods and lifters. I did this to prevent failure down the road.Well today I installed the new Holly Carb And fired it up.I noticed that at idle in runs really ruff compared to a faster speeds.I installed a vacuum meter and it reads very erratic!!!Can anyone tell me the easiest way to find a vacuum leak I plugged all the lines and took the test again and still reads the same. But at a faster speed it seems to smooth out and the meter settles down.Its so bad I have no power brakes.Could this be a leak?or maybe my cam is off?timing? I m at a loss .Dose anyone want an old Ford . Any advice would be helpful .Thanks
Laughing Gas
Joined: Jan 2002
Posts: 1,008
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From: Tucson
It sure does sound like a vacuum leak. You can spray WD-40 or something around all of the sealing surfaces and see if it smooths out when the leak is covered. I know these manifolds are famous for leading at the front and back surfaces and most guys use RTV or some other kind of sealer rather than the gaskets that come with the kit. Good luck and let us know.
jor
jor
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Joined: Nov 2002
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From: Clayton CA
The erratic vacuum reading is actually a diagnostic. The chart that came with my vacuum gauge gives several possibilities depending on the severity and pattern (if any) of the fluctuation. For example:
"Needle vibrates rapidly at idle, steadies as rpm is increased" means worn valve guides. There're a few others if that doesn't sound quite right.
Vacuum leaks, in general, just result in a low, but steady, vacuum reading.
Problems with the intake end seals don't result in vacuum problems, but will give you oil leaks.
Hope this helps.
"Needle vibrates rapidly at idle, steadies as rpm is increased" means worn valve guides. There're a few others if that doesn't sound quite right.
Vacuum leaks, in general, just result in a low, but steady, vacuum reading.
Problems with the intake end seals don't result in vacuum problems, but will give you oil leaks.
Hope this helps.
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Leading towards Valve guides
I fine tuned the Carb ,Sprayed wD 40 all around any manifold surface and still no change. I cant shift the tranny to move the truck with out it stalling.I digging deeper into to problem .Thanks for all the advice.PS it starting to burn oil out the exhaust at idle.
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Yes Its all good
I torqued all the bolts in order and made sure all the gaskets are sealed. the WD4o test confirmed it. The vacuum reading is on the low side . I leaning towards a faulty carb base gasket. That was the only gasket that is not new.
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Post Fiend
Joined: Mar 2004
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From: Union, Washington
Originally Posted by Lionsfan
I torqued all the bolts in order and made sure all the gaskets are sealed. the WD4o test confirmed it. The vacuum reading is on the low side . I leaning towards a faulty carb base gasket. That was the only gasket that is not new.
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IF you used an Edelbrock intake, and the stock bolts, the bolts are bottoming out in the heads and the intake is not receiving the correct torque to keep it seeled.
The burning oil at idle thing is oil that is getting sucking into the gap between the intake and the heads.
If one runner is leaking much more than the other ones, you will probably see the vacuum guage bounce, like you're seeing.
If you are using stock bolts for the intake, a washer (or two) will get rid of that.
To test the theory, remove the intake bolts, add two washers and re-install and torque them down (the right way, criss-cross pattern). You should see an immediate improvement in idle quality. If you don't, something else is happening. If it does help, remove the intake, get new gaskets and do it all over again
The burning oil at idle thing is oil that is getting sucking into the gap between the intake and the heads.
If one runner is leaking much more than the other ones, you will probably see the vacuum guage bounce, like you're seeing.
If you are using stock bolts for the intake, a washer (or two) will get rid of that.
To test the theory, remove the intake bolts, add two washers and re-install and torque them down (the right way, criss-cross pattern). You should see an immediate improvement in idle quality. If you don't, something else is happening. If it does help, remove the intake, get new gaskets and do it all over again
Postmaster
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I had the same problem on my 390 FE engine. I had installed a 390 Edelbrock intake. The engine ran good probably for about 2000 miles and the engine performance became sluggish. The engine block had been milled and the engine heads also were milled. Never could get the Edelbrock manifold to seal properly against the head. Removed the Edelbrock Intake Manifold and installed the good old Ford Manufactured Intake Manifold. Problem was solved.
Ever since then, I stop buying EDELBROCK PARTS.
Ever since then, I stop buying EDELBROCK PARTS.
Post Fiend
Joined: Mar 2004
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From: Union, Washington
Originally Posted by 1975Ford
I had the same problem on my 390 FE engine. I had installed a 390 Edelbrock intake. The engine ran good probably for about 2000 miles and the engine performance became sluggish. The engine block had been milled and the engine heads also were milled. Never could get the Edelbrock manifold to seal properly against the head. Removed the Edelbrock Intake Manifold and installed the good old Ford Manufactured Intake Manifold. Problem was solved.
Ever since then, I stop buying EDELBROCK PARTS.
Ever since then, I stop buying EDELBROCK PARTS.
Postmaster
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Originally Posted by Bear 45/70
On the FE engines, when the heads are cut more than a leveling cut you also have to cut the intake manifold or it won't seal right. It is in the Ford Perfomance Books and tells you how much to cut off the intake for how much the head were cut. All you need to do was cut the Edelbrock manifold the right amount. The problem wasn't Edelbrocks fault.
post # 7 by "Beemer Nut"
I have the Edelbrock intake in the shed. I probably give the Edelbrock associates a call, to check if this was one of the defectives intakes. Then if the intake manifold is good. I'll cut off the intake manifold as the cylinder heads were cut.
Bear 45/70 thanks
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