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?