Manifold vacuum test
A manifold vacuum test aids in determining the con-dition of an engine and also in helping to locate the cause of poor engine performance. To check manifold vacuum
1. Operate the engine for a minimum of 30 minutes at 1200 r.p.m.
2. Remove the plug or power brake line at the rear of the Intake manifold and install an accurate, sensitive vacuum gauge.
3. Operate the engine at recommended idle r.p.m. with the transmission selector lever in neutral.
4. Check the vacuum reading on the gauge.
Test conclusions
Manifold vacuum is affected by carburettor adjustment, valve timing, ignition timing, the condition of the valves, cylinder compression, the condition of the crankcase ventilation system, and leakage of the manifold, carbu-rettor, carburettor spacer or cylinder head gaskets.
Because abnormal gauge readings may indicate that more than one of the above factors are at fault, exercise caution in analyzing an abnormal reading. For example, if the vacuum is low, the correction of one Item may increase the vacuum enough so as to indicate that the
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trouble has been corrected. It is important, therefore, that each cause of an abnormal reading be Investigated and further tests conducted, where necessary, in order to arrive at the correct diagnosis of the trouble.
The table below lists various types of readings and their possible causes.
Allowance should be made for the effect of altitude on the gauge reading. The engine vacuum will decrease with an increase in altitude.
Camshaft lobe lift
1. Remove the air cleaner and the valve rocker arm cover(s).
Remove the rocker arm stud nut, fulcrum seat and rocker arm.
Make sure the push rod is in the lifter cup. Install a dial Indicator In such a manner as to have the ball socket adaptor of the Indicator on the end of the push rod and in the same plane as the push rod movement (Fig. 13).
2. Install an auxiliary starter switch. “Bump” the crankshaft over until the lifter is on the base circle of the camshaft lobe. At this point, the push rod will be in its lowest position.
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