Yamaha RS90K, Service Manual

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2-61
INSP
ADJ
CARBURETOR TUNING
Poor performance at
mid-range speeds:
• Momentary slow
response to the throt-
tle
• Poor acceleration
Clogged or loose pilot jet • Remove the pilot jet, clean it with compressed
air, and then install it.
• Make sure that the pilot jet is fully tightened.
Lean mixture Overhaul the carburetors.
Poor performance at
normal speeds:
• Excessive fuel con-
sumption
• Poor acceleration
Clogged air vent Remove the air vent hose and clean it.
Clogged or loose main jet • Remove the main jet, clean it with compressed
air, and then install it.
• Make sure that the main jet is fully tightened.
Overflow Check and, if necessary, clean the float and float
valve.
Poor performance at
high speeds:
• Power loss
• Poor acceleration Starter valve is left open Return the starter lever to its seated position so
that the starter valve is fully closed.
Clogged air vent Remove and clean the air vent.
Clogged or loose main air jet • Remove the main jet, clean it with compressed
air, and then install it.
• Make sure that the main jet is fully tightened.
Clogged fuel line Clean or replace the fuel line.
Dirty fuel tank Clean the fuel tank.
Air leaks into the fuel line Tighten or replace the fuel line joint.
Low fuel pump performance Repair or replace the fuel pump.
Clogged fuel filter Replace the fuel filter.
Clogged intake Remove any obstructions (e.g., ice).
Abnormal combustion:
• Backfiring Lean mixture Clean and adjust the carburetors.
Dirty carburetors Clean the carburetors.
Dirty or clogged fuel line Clean or replace the fuel line.
Overflow:
• Poor idling
• Poor performance at
low, mid-range, and
high speeds
• Excessive fuel con-
sumption
• Hard starting
• Power loss
• Poor acceleration
Clogged air vent Clean the air vent.
Clogged float valve • Disassemble and clean the float valve.
• Do not scratch the valve seat.
Scratched or unevenly worn
float valve or valve seat • Clean or replace the float valve and valve seat.
• The valve seat and body must be replaced as a
set.
Broken float Replace the float.
Incorrect float level Check and, if necessary, replace the following
parts:
• Float tang
• Float (entire assembly)
• Arm pin
Trouble Diagnosis Adjustment