Onan MJA Series Service Manual page 75

73
3.7. Cylinder Heads
and
Valves
Each cast iron cylinder head assembly contains valves, valve seat
inserts and guides, rocker arms, and spark plugs. The valve assem ­
blies are operated by pushrods running through the cylinder block to
the camshaft. Exhaust valves are stellite faced, and ride on stellite
seat inserts; all valves have release type rotators.
3.7.1 MAINTENANCE
Check the valve clearances at regular intervals (see service chart for
1.1:
periods). In addition, clean tpeeorilbu.stion chambers and inspect the
valve and valve seats at regd~r peri~~\
·':".;.·~r'''''''~~,.-:i·~··------------
Always check the valve clearance
while the engine is at
roorriitem~
perature (approximately
700¥)~
Turn the flywheel so the cylirider
for the valve to be checked is 10
0
to 45
0
past the TDC position of
the compression stroke. Adjust
the clearance, with the rocker
arm nut,. (Fig 3.7 .A). For valve
clearance values, .see the valve
clearance table (Fig 3. 7 • B).
INTAKE
EXHAUST
MJA WB
Prior to Spec D
Begirming Spec D
.019
.010
.014
.023
.010
.018
WC -
.015
.020
FIG 3.7.B - VALVE CLEARANCE CHART
(Engine at 70
0
F Room Temperature)
3.7.2 TESTING
I)
\
The cylinder cOPlpreSsion test can be used to determine the condition
of valves, pistons, piston rings and cylinders .
To check compression, run the engine until thoroughly warm. Stop
it,. .
and remove all spark plugs. Insert the compression gauge in a spark
plug hole, crank the engine, and note the reading. To check for piston
blOW-by, squirt a small amount of SAE 50 oil into the cylinder and
repeat the check. An increase in compression with oil in the cylinder
indicates piston blow·by.
Compression of a standard new engine cranking at about 300 rpm is
about 110
DSi. Comnrp~~l()n ~hr11l1rl hI'>
f",i,..hr 1l,."f,............
~
....
~_nl1 ••••• a\..
'--'"
FIG 3.7. A - ADJUSTING VALVE
CLEARANCE