COMPRESSION CHECK.

g . A r e a d i n g o f m o r e t h a n 1 0 p o u n d s above

n o r m a l indicates carbon or lead deposits in the

cylinder.

h. A reading of more than 10 pounds below

normal indicates leakage at the head gasket, rings,

or valves.

i. A low even compression in two adjacent

cylinders indicates a head gasket leak. This should

be checked before condemning the rings or valves.

j. To determine whether the rings or the valves

are at fault, squirt the equivalent of a tablespoon

o f heavy oil into the combustion chamber and

Figure 4-43. Compression Check

repeat the compression test. The oil will

temporarily seal leakage past the rings. If the same

reading is obtained, the rings are satisfactory, but

A cylinder

4 - 4 0 . COMPRESSION CHECK.

t h e valves are leaking. If the compression has

compression test aids in determining the condition

i n c r e a s e d 10 pounds or more over the original

of the valves, rings and head gasket.

reading, it indicates there is leakage past the rings.

a. Be sure the battery is good. Operate the engine

4-41. SPARK PLUG CHECK. Under normal

u n t i l normal operating temperature is reached.

c o n d i t i o n s , these spark plugs will give long life

T u r n the ignition switch off. Loosen the spark

performance with the normal maintenance listed in

plugs, blow out any dirt in the spark plug wells,

this manual. The spark plugs should be cleaned,

then remove the plugs.

tested and gapped at the recommended intervals.

b. Set the accelerator in the wide open position

a . Loosen the spark plugs one full turn, then

and be sure the choke is wide open.

b l o w any accumulation of dirt out of the spark

plug wells before completing the removal.

c. Remove the coil high tension lead in the

distributor, and ground it securely to the engine.

b. Examine firing end of plug carefully and

I n s t a l l a compression gauge in No. 1 cylinder.

compare findings with figure 4-44. Careful analysis

Crank the engine until the gauge registers a

of spark plug deposits can reveal a great deal about

maximum reading and record the reading and note

engine operating conditions.

t h e number of compression strokes required to

obtain this reading. See figure 4-43.

c . Remove carbon and other deposits from the

d. Repeat the test on each cylinder, cranking the

threads with a stiff wire brush. Any deposits will

same number of times that were needed to obtain

retard the heat-flow from the plug to the cylinder

the maximum reading on No. 1 cylinder.

head,

causing

spark plug overheating and

pre-ignition.

e . During the compression test, the indicated

p r e s s u r e should rise evenly on each succeeding

d. Clean any heavy carbon deposits from the

stroke until the maximum reading is obtained. If

inside of the plugs with a thin-bladed knife, then

the pressure rise is erratic, or fails to rise on any

finish cleaning them with an abrasive-type cleaner.

stroke, a sticky or stuck valve is indicated.

Use the cleaner sparingly, as excessive abrasive

b l a s t i n g may damage the porcelain around the

f. The pressure should be approximately 150-200

center electrode. If the porcelain is badly glazed or

pounds. However, the compression of all cylinders

should be uniform within 20 pounds.

eroded, replace the spark plugs.

4-32