Hey Cos, Have you ever had a vacuum gauge on the motor? Carb? Main intake vacuum port? Just to check to see where your at? Have you tried spraying carb cleaner at the intake port's, where the manifold is bolted to the head? If the engine is painted real nice, I will say it may take some paint off,
anyway, spray some at the gasket mating surface's, and while engine is running, if engine idle speed's up (rev's up), then you've found the area where there is a leak. It is also very possible, that it could be leaking some vacuum, on the underside of intake too, and then you would at least have to pull the intake to check it out. Also, spray around the base of the carb. These are at least the area's I would check out first.