Ford Flathead V8 weird vacuum question

[Concept Cars]

The engine runs badly and there's an unusual situation with vacuum under the intake manifold, in the lifter valley. When I remove the oil fill breather and put my hand over it, it nearly sucks my hand in. When I spray carb cleaner down the oil fill, the RPM increases. Spraying carb cleaner around the manifold makes no difference. The compression is 110-120 in all cylinders except #5 which is 80-lbs. It doesn't idle or accelerate smoothly, it runs like there's a bad vacuum leak. It idles (roughly) with about 5-hg of vacuum, and the needle bounces a bit. The engine is a 59AB, I believe it has low hours, but hasn't been driven in many years. I swapped the intake manifold and installed a new gasket after cleaning the mating surfaces. I tried different carbs, adjusted the timing, but no difference.
Where do I begin to diagnosis why there's vacuum under the intake manifold?
Thank you for any advise or suggestions.

[drolston]

With good (flat, no cracks) mating surfaces and a good intake manifold gasket, that leaves horribly worn intake valve guide(s).

[JSeery]

I would suspect a crack or opening in the bottom of the intake manifold.

[Concept Cars]

Originally Posted by drolston

With good (flat, no cracks) mating surfaces and a good intake manifold gasket, that leaves horribly worn intake valve guide(s).

The manifold gasket and mating surfaces are perfect. But, I think worn intake valve guides might be a possibility. Wouldn't that show up on the compression test? I pulled all the plugs and tested each cylinder, they all pumped up to 110 - 120, except one cylinder #5 pumped up to only 80-psi.

[Tinker]

Valves would be closed. Blocking off air flow to the guides or intake valley.


The one cylinder might explain the bouncing vacuum reading. Could be hung up and not closing giving you a low compression check. Maybe burnt, but on a new engine I would think not. Probably due to more a non running issue then a machined issue. Never know. Spray some lube in that cylinder before tearing down. L heads used leaded gas to help lube the upper engine. OHV had oil piped up. Not saying to use leaded gas.


I'm going to just guess its a 1 or 5 hole?


https://www.fordbarn.com/forum/showt...t=vacuum+chart
Post 11#


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[Karl Wolf]

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Try looking for a hole in the intake manifold, between the heat riser and the intake under the carburetor.
I bought a used intake manifold from a shop that had a hole the size of a penny there.
This made for a huge vacuum leak, but from the exhaust, not inside the engine.

The vendor understood the problem and took back the part.

Karl

[supereal]

Check the vent port under the front of the oil pan to be sure it is clear. Otherwise, there is no way for the cap on top of the fuel pump stand to work. Sometimes a pan gasket incorrectly blocks the triangular port.

[sunshine]

I have the same problem with my 39 l I have tried to find the vacuum leak as it shows about 12hg with a vibrating needle . My engine builder says it can’t be valve guides on a 300 mile engine . But how about bad new guides .? He says do a leak down test. That is my next step when I am able. sunshine