How To Diagnose Common Engine Problems With A Vacuum Gauge

power and economy depend on the basic
mechanical condition of the engine to perform some engine diagnostic test
you will need a vacuum gauge no amount of carburetor ignition or
exhaust work will make up for burn valve leaking head gasket or worn piston
rings the following tests and inspections will help you locate
mechanical problems in the engine that may cause rough running lack of power
or hard starting begin by connecting the gauge to a full manifold vacuum source you could use a
vacuum T on an existing line or pulla lines such
as one that leads to a transmission or missions control unit don’t use the
side port located on the metering block of any Holley carburetor this is a timeed port that will only show
full vacuum at higher RPM’s from here you can either
select one of the gauge numbers if you’re familiar with the video or just wait for the guided tour all
gauge reading descriptions keep in mind when we refer to a normal
vacuum reading we’re talking about a reading on an
engine in sound mechanical condition that’s been checked and a sea level
elevation for elevations around 3,000 feet and above your gauge readings may be
slightly lower but will have the same reactions are shown the following are some typical gauge
readings and what each indication means a steady reading between 16 and 22
inches at idle is normal for most mildly cam to stock
engines radically camed engines will inherently
have a lower and less steady reading a normal range at idle with sporadic drops
below normal might indicate some valves are sticking a normal range at idle with a needle
vibration of about two inches may indicate an
ignition problem check the spark plugs in gaps the plug
wires the distributor cap and rotor the
distributor dwell setting and the coil connection itself a steady reading that’s slightly higher
than a previously noted normal reading might be caused by a
dirty air filter or and overly advanced ignition timing a steady reading at three to 12 inches
lower than normal may indicate one or more of the
following conditions an intake carburetor or vacuum system air leak a late
ignition or cam timing setting or worn piston rings

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