Yamaha RS90K Manuel D'atelier page 165

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