carbs

[Ian Williams]

Have you checked the manifold thoroughly for cracks holes or distortion? Everything you describe still points to weak mixture, if you are 100% certain that the carb is clear of blockages, has the correct jets, fuel level and fuel supply then your problem has to be an air leak, which can only really come from the manifold, either through distortion, bad gasket or a hole or crack of some sort. Take the manifold back off, with a straight edge ensure it is flat where it meets the block, check the face of the block is also flat and do the same on the carb flange and corresponding manifold face. correct any issues, then remake both joints with new gaskets and a decent flange sealant such as Loctite 518, Hylomar or similar. I would also inspect the manifold very closely for any cracks, holes or other flaws or damage before refitting.
You mentioned valves, what makes you suspect this? I am assuming that you have checked compression's are good and even across all cylinders. Is there a history of a previous owner rebuilding the engine? I would not be immediately suspecting this but it is just possible the cam timing is way off, that said I am far more inclined to be suspecting weak mixture. You said that there was a lot of fuel in the bottom of the manifold after revving with the air intake blocked, this would be expected as the engine vacuum would be pulling almost neat fuel through the carb, but air must be getting in somewhere or it would simply flood and die when you block the intake.