If you have 11" of vacuum in the intake manifold, on a carb enginge, at idle, then your engine is 'sick'. Have you tried spraying solvent around the carb gaskets, to see if the rpm's increase?

My understanding of vacuum, at idle, is a minimum of 16",
and the closer to 19" the better. You can disconnect the 'retard' vacuum line on the dizzy, as a source.

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1970 280S M130 engine- good runner
1971 250 M130 engine- #2 rod bearing, gone
1971 280SE (blown engine,parts car)
1977 German 280S W116-only 33 years old