[Capt Kirk]

Three different sources (Haynes, Chiltons and Equus) state the following for low but steady vacuum.

1) a lower reading indicates possible incorrect timing, incorrect valve timing or adjustment, incorrect setting of idle mixture, worn piston rings, or leak in intake manifold.

2) gauge reading low (15-20 in Hg) but steady indicates late ignition or valve timing, low compression, stuck throttle valve, leaking carburetor or manifold gasket.

3) low steady reading usually indicates leaking gasket between intake manifold and carburetor or throttle body,
leaky vacuum hose, or incorrect camshaft timing.