Yamaha RS90K Manuel D'atelier page 164

Spark plug color
Light tan or gray
Dry black or fluffy
deposits
White or light gray
White or gray insulator
with small black or
grayish brown spots
and electrodes having a
bluish-burnt appear-
ance
Melted electrodes and
possibly a blistered
insulator
Metallic deposits on
insulator
Troubleshooting
Trouble
Hard starting
Poor idling:
• Poor performance at
low speeds
• Poor acceleration
• Slow response to
throttle
• Engine tends to stall
CARBURETOR TUNING
Main jet selection chart
Diagnosis
Carburetors are tuned prop-
erly.
Mixture is too rich.
Mixture is too lean.
Mixture is too lean. The piston
is damaged or seized.
Mixture is too lean. The spark
plug melted.
Diagnosis
Insufficient fuel
Excessive use of the starter or
choke
Fuel passage is clogged or
frozen
Overflow
Improper idling speed adjust-
ment
Damaged pilot screw
Clogged bypass hole
Clogged or loose pilot jet
Air leaking into the carburetor
joint
Defective starter valve seat
Overflow
Replace the main jet with the next smaller size.
Replace the main jet with the next larger size.
Replace the piston and spark plug.
Tune the carburetors again. Begin with low-speed
tuning.
Check the piston for holes or seizure.
Check the cooling system, gasoline octane rating
and ignition timing. After replacing the spark plug
with a colder type, tune the carburetors again.
Begin with low-speed tuning.
Add gasoline.
Return the starter lever to its seated position so
that the starter valve is fully closed.
• Check and, if necessary, clean the fuel tank air
vent, the fuel filter and all of the fuel passages.
• Check and, if necessary, clean the carburetor
air vents, fuel passages and the float valve.
• Clean the float chamber of any ice or water.
Adjust the fuel level.
Adjust the engine idle speed.
Refer to "Low-speed tuning".
Replace the pilot screw.
Clean the bypass hole.
• Remove the pilot jet, clean it with compressed
air and then install it.
• Make sure that the pilot jet is fully tightened.
Retighten the clamp screws on the carburetor
joints.
Clean or replace the starter valve seat.
Adjust the fuel level.
2-60
INSP
ADJ
Remedy
Adjustment