Engine Tests - TM-10-3930-621-120112

3-27. Engine Tests

b. Removal.

(1) Refer to paragraph 3-29 and remove the

a. General. The following tests aid in checking the

carburetor.

condition of the engine and aid in determining the

(2) Refer to paragraph 3-35 and disconnect the

repairs that may be required and also the extent of

exhaust pipe from the outlet flange.

those repairs.

(3) Remove the hose clamps (fig. 3-12) and dis-

b. Compression Test. The compression test helps

connect the hose from the vent valve.

to determine the condition of rings, valves, and head

(4) Remove the vent valve and elbow from mani-

gasket. Check compression as described below.

fold.

(1) Start and operate engine at idle speed until

normal operating temperature (fig. 2-2, is reached.

(2) Shut off engine and leave ignition switch in

off position.

(3) Set carburetor throttle and choke controls in

the full open position.

(4) Remove the spark plugs. Install a compression

gage tightly in number one spark plug hole.

(5) Crank the engine with the starter to allow six

compression strokes on the gage. Note the readings.

(6) Repeat the test with the gage for each of the

remaining cylinders.

(7) Readings on gage should not vary more than

10 pounds. Starting speed normal compression

should be 120 to 125 psi.

(8) If any of the cylinders have low readings,

retest as follows:

(9) Add oil through the spark plug holes in an

amount sufficient to seal the piston rings.

(10) Crank the engine over five or six times to

allow oil to work down around rings and install gage

as above. Repeat test to obtain another compression

reading.

(11) If compression increases, defective or worn

pistons or rings are indicated.

(12) If compression does not increase, defective

Figure 3-12. Mainifold, installed view.

valves are indicated.

(13) If two adjacent cylinders show low readings,

(5) Remove six nuts and lock washers securing

a defective cylinder head gasket is indicated.

manifold to engine block and remove manifold and

(14) Install spark plugs and operate engine to

gaskets from engine.

check operation.

c. Cleaning and Inspection

c. Vucuum Test. Start engine and allow to reach

(1) Scrape all carbon and deposits from mani-

operating temperature. Shut off engine and discon-

folds.

nect vent valve hose and fitting from manifold (para

(2) Inspect manifolds for cracks.

3-26). Install a vacuum gage in intake manifold at

(3) Inspect vent valve hose for damage and

fitting opening. Vacuum test the engine as follows:

deterioration. Flush the valve with cleaning com-

(1) Start the engine and operate at idle speed.

pound, solvent (Spec. P-S-661) and dry thoroughly.

Observe the vacuum gage.

(4) Replace vent valve at least once a year.

(2) Normal vacuum of approximately 18.3 inch

d. Installation.

suction pressure should be indicated. Open throttle

(1) Install manifolds (fig. 3-12) with new gaskets

and observe gage. A drop of about 3.4 inches should

and secure with nuts and lock washers.

be noted. When throttle is closed, suction pressure

(2) Install elbows and connect vent valve (fig.

should increase to about 24 inches.

3-12) and host to manifold and block.

(3) A steady pressure of 15 to 16 inches indicates

(3) Refer to paragraph 3-29 and install the car-

pistons, piston rings, or lubricant are in poor condi-

buretor.

tion. Slight needle fluctuation indicates late ignition

(4) Refer to paragraph 3-35 and connect the

timing. Refer to paragraph 3-44 to time distributor

exhaust elbow to the manifold.

3-16