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Wisakedjack, Sorry it took so long for the light to finally come on, but, since we're dealing with a Catalina-30 here, you may have a vacuum leak at the back end of the manifold. Here's how it goes. The hot section of a c-30's exhaust runs laterally from the manifold over to the top of the water-lift/muffler, where it drops down and connects, normally with a short length if rubber hose, to the inlet on the top. Engine torque, over time, tends to either crack the water-lift or to loosen up or actually crack the flange at the back of the manifold. Neil and I replaced a manifold on an A4 a couple of years ago that had completely cracked off. The manifold had rusted through and the crack went undetected until after it was removed. Recently I rebuilt my hot section and replaced my manifold because of a crack in the flange area. One way to check for a vacuum leak at the manifold gasket is to spray some WD-40 between the head and the manifold while the engine is running. If there is a leak, the engine rpms should change and some white smoke should show up in the exhaust. I realize that you had recently removed the head, but didn't notice if you removed the manifold or not.
Tom
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