166 M A I N T E N A N C E - E N G I N E
i t s center portion. One hole should be dbout 6 mm
in diameter, the other about 3 mm i n diameter.
The plate must be oil resistant, about 120 nlm
square, and at ledst 3 mm thick.
Measuring Plastic Plate
3 5 m m
burette or syringe which i s calibrated at
one-cc or smaller graduations.
Fill i t with thin oil.
Prior to the combustion chamber volume measure-
ment, clean o f f any carbon on the combustion chamber,
and remove any gasket flakes on the cylinder head
The standard spark plug should be
installed in the chamber to be measured.
The valves must seat well to prevent the oil
from leaking out.
Apply a thin coat of grease to the cylinder head
mating surface and pldce the plastic plate over the
cylinder head combustion chamber, fitting its small
hole near the edge of the combustion chamber.
A. Plastic Plate
C. Small Hole
B. Large Hole
Place the cylinder head on
level surface. Through
the large hole, fill the combustion chamber with light
oil such as 2-stroke oil or mission oil until the chamber
is completely filled but not overly.
Tilt the cylinder
head slightly so that air bubbles come out through
the small hole. The oil should just rise to the bottom
edge o f the holes in the plate.
The amount o f oil used to fill the chamber i s the
combustion chamber volume.
Table H I 4
Combustion Chamber Volume
I f the combustion chamber volume is too small, i t
i s possible that the cylinder head was modified for
higher compression. Make sure that all carbon deposits
have been cleaned out o f the chamber.
I f the combustion chamber volume i s too large, i t
is possible that the valves and valve seats have been
resurfaced so much that the volume is increased. Make
sure that the spark plug i s the standard type and that
i t i s fully tightened.
Valve, Valve Guide, Valve Seat
Valve face deformation or wear, stem bending or
wear, and valve guide wear can cause poor valve seating.
Poor seating can also be caused by the valve seat itself,
i f there i s heat damage or carbon build-up. The result o f
poor valve seating i s compression leakage and a loss o f
In addition, valve and valve seat wear causes deeper
valve seating and
decrease in valve clearance.
sufficient clearance upsets valve timing and may even-
tually prevent the valve from seating fully. So that wear
never progresses this far, adjust the valve clearance in
accordance with the Periodic Maintenance Chart (Pg.
Visually inspect the valve face, and replace the valve
i f i t shows deformation or uneven wear.
Measure the thickness o f the valve head using vernier
calipers, and replace the valve i f the thickness is under
the service limit.