Onan MJA Series Service Manual page 51

49 I G NIT ION S Y S T E M
2. Run the plant at rated speed. Aim the flashing timing light toward
the flyw hee l.
3.
If
the timing pointer on the gear cover indicates 250 BTC, timing is
correct. Otherwise, loosen the distributor body clamp and rotate
the distributor body to adjust timing. If timing is early (25
0
mark
to the right of the pointer) rotate the distributor clockwise to retard
the ignition point. Tighten the distributor in its mount and recheck
timing with the light.
4. The no. 1 set of breaker points fires cylinders 1 and 4. Breaker
point set no. 2 fires cylinders 2 and 3 and should be properly timed
to open if the gap is correct (.017 - .022 in.). The timing may be
checked by placing a timing mark on the flywheel 180 0 from the 25
0
BTC mark (155° ATC). Then repeat steps 1 - 3 substituting cylin ­
der 2 fOr cylinder 1 and the. new timing mark for 250 BTC.
NOTE: If the relative position of the timing marks doesn't remain
steady, the distributor is probably defective. This can be
caused by pitted or misaligned breaker points, incorrect
breaker pOint spring tension, WOrn or loose breaker plate
or a worn distributor shaft or bushing.
3.3.4.3 Distributor. - The distributor serves 3 functions.
It
contains
and opens the breaker pOints at the proper time, contains an automatic
spark advance mechanism, and serves the two coils which supply high
voltage to the spark plugs.
A. Testing. - The cam dwell, firing a single point, is a mean of 321
degrees. Since this is not readable on most meters the point opening
should be adjusted. 017 - .022 inches. See Sect 3.3.4.1.
Thoroughly inspect the breaker points and check to be sure the movable
contact turns freely on its pivot.
Using aspring scale, measure the tension of the points as they break
contact. Tension should be 17 to 20 ounces. If it is greater, it causes
excess wear;
if
less, the contact will bounce.
See
step 9, sub sect D,
. distributor assembly,
HOOK OVER
BREAKER ARM
AT CONTACT
to adjust tension.
SPRING
SCALE---......
(I
~~~~~_~':~"._~':...~
_ _
FIG 3.3.J - :rvlEASURING CONTACT SPRING TENSION