Yamaha MM700F, Service Manual

Page: 124 / 589

2-45
INSP
ADJ
Remedy
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.
Diagnosis
Carburetors are tuned
properly.
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.
Main jet selection chart
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
appearance
Melted electrodes and
possibly a blistered
insulator
Metallic deposits on
insulator
CARBURETOR TUNING
Troubleshooting
Trouble
Hard starting
Poor idling:
•
Poor performance at
low speeds
•
Poor acceleration
•
Slow response to
throttle
•
Engine tends to stall
Diagnosis
Insufficient fuel
Excessive use of the
starter or choke
Fuel passage is clogged
or frozen
Overflow
Improper idling speed
adjustment
Damaged pilot screw
Clogged bypass hole
Clogged or loose pilot jet
Air leaking into the carbu-
retor joint
Defective starter valve
seat
Overflow
Adjustment
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.