Yamaha XZ550RJ 1982 Service Manual page 4

LIQUID COOLING SYSTEM
Downdraft carb
The induction system on this motorcycle features downdraft carburetion. The 34 mm (1.34 in) Venturis are
positioned so that the incoming air takes a virtually straight path through the carbs and the intake manifolds to
the combustion chambers; intake efficiency is remarkably high. An accelerator pump is installed in the carbu-
retor assembly to eliminate hesitation when the throttle is opened at low engine speeds. A fuel pump delivers
gas from the tank to the float bowls, and a regulator sends excess fuel through a circuit back to the fuel pump
inlet.
DOHC 4-valve heads
Each cylinder head has dual overhead camshafts, and each camshaft acts directly on top of the valves; there
are no pushrods or rocker arms. Both heads have four valves, two intake and two exhaust, instead of one in-
take and one exhaust valve. By using two slightly smaller valves in place of each large valve, port area is sig-
nificantly increased. This provides a much greater volume of mixture and exhaust flow. In addition, the smaller,
lighter valves are easier to control at high engine speeds; the valves can follow the cam profiles more closely
to resist "floating" and allow a 9,500 r/min red line.
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