Looking at the following guidelines on
this page:
Quote:
Gauge will indicate loose valve stem guides when gauge pointer pulsates rapidly at idling speed. This rapid pulsation disappears with the increasing speed of the engine.
Gauge will indicate a choked muffler when engine is speeded up several times in succession and the gauge pointer drops slowly. A clear muffler is indicated by a quick drop to zero.
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So the drop to 0 seems like it's normal behavior, and the pulsating at idle could indicate loose valve stem guides.