CHECKING IGNITION SPARK.

TM 55-4920-335-14

g. When magneto is properly timed, the impulse

metal part of engine as shown in figure 4-4.

coupling will snap when the DC and X marked vane of the

flywheel lines up with mark on flywheel which should indicate

b. Turn engine over slowly using hand crank and watch

the centerline of number 1 and 3 cylinders. Check this by

for spark discharge which should occur during the cycle, at the

turning crankshaft over by hand. The impulse will also snap

instant the impulse coupling on the magneto snaps. Repeat

every 180 degrees of flywheel rotation thereafter.

this check with each ignition cable.

h. The proper spark advance is 23 degrees. Timing may

c. If there is a weak spark, or none at all, check magneto

be checked with a neon light, the running spark advance is

breaker point opening (paragraph 4-13). If this does not

indicated by a 1/8 inch hole on flywheel shroud, 23 degrees

remedy trouble it may be necessary to install a new condenser.

before vertical centerline of number 1 and 3 cylinders. Whiten

end of vane marked X with chalk or paint. The magneto

4-13. ADJUSTING ENGINE MAGNETO BREAKER POINTS.

rotates at crankshaft speed in clockwise direction as viewed

If ignition spark becomes weak after continued operation, the

from driving gear end of magneto. The magneto distributor

breaker points may have to be readjusted, resurfaced, or

rotor turns at half engine speed.

replaced, as follows:

4-12.

CHECKING IGNITION SPARK.

If difficulty is

a. Remove magneto end cover and examine points. If

experienced in starting the engine or if engine misfires, the

there is evidence of pyramiding or pitting, resurface points

strength of the ignition spark may be tested as follows:

using a small tungsten file.

a. Disconnect number 1 ignition cable from spark plug

and hold terminal 1/8 inch away from air shroud or any other

Figure 4-3. Engine Ignition Timing

4-5