Figure 4-3. Compression test,

Note. All cylinders must be tested the same number of

and block in an open position.

strokes to assure accurate readings.

(7) Insert compression tester in spark plug

hole (fig. 4.3) using an adapter if necessary.

(9) Proper cranking compression pressure is

110 psi to 140 psi.

(8) Turn ignition switch on and depress starter

switch to crank engine for at least four compression

( 1 0 ) If the readings are below normal or

uneven, place the equivalent of a tablespoon of OE

strokes. A starter

control cable may be used

30 oil in each cylinder and retest compression.

if available, Note pressure reading on first full

stroke as well as on final stroke. See test indications

Note. Be certain oil is deposited on piston head,

below.

Figure 4-3. Compression test,

c. Test Indications.

timing test is to determine the firing point with

respect to piston position, and to determine the

(1) N o r m a l . C o m p r e s s i o n b u i l d s u p q u i c k l y

and evenly to specified compression in each

need for ignition timing adjustment,

cylinder, v a r i e s less than 15 pounds between

b. Procedure.

highest and lowest reading cylinders.

(1) Remove spark plug wire from number one

spark plug, and install adapter (fig.

(2) Ring trouble. Compression low on first

4-4) to connect timing light.

stroke, tends `to build up on following strokes but

Caution: Use adapter. Do not puncture

does not reach normal. Improves considerable with

insulation on spark plug wire to make con-

addition of oil.

nection.

(3) Valve trouble. L OW on first stroke does not

tend to build up on following strokes; does not

(2) Connect timing light spark plug lead as

improve much with addition of oil.

shown, and power leads to battery terminals.

(4) Leaking head gasket. Same reaction as

Note. If available equipment requires different power

connections, such as connection to a 115 volt source, use

valve trouble on two adjacent cylinders, usually

appropriate procedures.

accompanied by indications of water in cylinders

and / or oil in cylinders.

(3) Start engine and, at 450 rpm idle speed,

(5) Carbon deposits. C o m p r e s s i o n p r e s s u r e

d i r e c t flashing beam of timing light at top of

considerably higher than specified.

crankshaft pulley and pointer. If ignition timing is

Note. After completing compression test, reinstall

c o r r e c t , light flash will show scribed mark on

spark plugs, plug wires and coil to distributor lead. Return

crankshaft pulley aligned with pointer on gearcase

choke and throttle controls to normal.

cover as plug fires. If timing is not correct, refer to

4-23. Engine Ignition Timing Test

paragraph 4-49 e for adjustment.

a. General. The purpose of the engine ignition

4-7