Yamaha XV16AL Service Manual page 11

Hydraulic valve lifters
Since the hydraulic valve-lifting mechanism maintains a valve clearance of zero, periodic
valve clearance adjustments are unnecessary.
The advantages of this system as compared to conventional techniques include the follow-
ing: mechanical noise is reduced, the camshaft action on the valves remains unaffected by
engine speed or temperature, and the valve timing is kept stable.
1
The hydraulic valve-lifting system functions as follows:
1. As the camshaft rotates, the valve lifter is pushed up by the passing cam lobe.
2. Since the check valve 4 prevents the engine oil contained inside the high-pressure cham-
ber from escaping, the plunger 1 moves up along with the valve lifter body 8 and pushes
up the push rod, causing the valve to be lifted.
3. As the camshaft continues to rotate, the valve lifter moves back down to its original posi-
tion, where it remains while the cam heel passes.
When a positive valve clearance is caused by either heat expansion of the cylinder head or
engine oil leaking from the valve lifter during stage 2, the plunger, which no longer receives
pressure from the push rod, is pushed up by the plunger spring 7. As a result, the valve
clearance is zeroed and engine oil is allowed to return to the high-pressure chamber from the
reservoir 2 through the check valve 4.
When, on the contrary, a negative valve clearance occurs (this is the case when the cam heel
is passing the valve lifter, but the rocker arm, pushed by the push rod, is lifting the valve), the
plunger 1 continues to receive pressure from the valve push rod. As engine oil contained
inside the high-pressure chamber leaks from the gaps between the valve lifter body 8 and
the plunger 1 as well as between the valve lifter body 8 and the check valve 4, the
plunger 1 moves down and the valve clearance is zeroed.
FEATURES
1 Plunger
2 Oil reservoir
3 Check valve spring
4 Check valve
5 Spring retainer
6 High-pressure chamber
7 Plunger spring
8 Valve lifter body
9 Oil supply inlet
0 Push rod cup
A Plunger retaining clip
B Valve push rod
1 - 5
GEN
INFO