RPM drops with Vacuum Gauge hooked up?

[SDMAN1981]

OK today I hooked up a Vacuum Gauge and the engine’s RPM dropped 200-300 RPM then when I hooked the distributor vacuum advance back up it went back up to normal RPM 950. Is that normal?

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[R.G.Young,Jr]

that is normal if you had disconnected the vacuum advance to hook up the vacuum gage. a vacuum gauge should be connected straight to the intake manifold or through a fitting that does not control a function of the engine at the time of testing i/e modulator valve, brake booster, vacuum reservoir tank etc, if you were working on a vehicle that has one, if not, you can tee into an existing manifold vacuum source to obtain the readings you need.

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