Yamaha YTZ250S 1986 Service Manual page 45

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TEMPERATURE _
• Change due to temperatue (at constant
atmospheric pressure and humidity)
Condition
Mixture Setting
will be make
Cold air lean rich
Warm air rich lean
Dry air
lean rich
Lowlands Standard
Highlands
rich lean
(Alt. above 1,500 m)
If your machine is too rich, it will :
•
Accelerate poorly;
•
Misfire;
•
Smoke excessively;
•
Foul spark plugs;
•
Have a "deep" exhaust note .
If your machine is too lean, it will:
•
Ping or rattle ;
•
Accelerate erratically;
•
Act like it's running out of fuel;
•
Run extremely hot.
• If you machine pings or rattles, make sure
the gasoline you are using is fresh and of
a sufficient octane rating. You might also
try different brands of high-octane gas.
Making setting changes
Carb setting changes are made by changing or
adjusting six carburetor components. The six
components and their relative positions in the
carb are shown in the illustration below.
The main jet should be increased or decreas-
@
ed one to five sizes by test until the engine gives
maximum power.
Symptoms of improper settings
If your machine exhibits one or more of the
symptoms listed below, it may need carb tun-
ing changes. Before attempting any changes,
however, make sure that everything else is in
good shape and tuned properly. Check the
condition of the spark plug, make sure the ig-
nition timing is correct, service the air filter pro-
perly, decarbonize the muffler and spark ar-
restor, etc. If your machine has run properly
at a certain track in the past and then starts
running poorly with the same carb settings, the
problem is almost certain to be elsewhere;
changing the carb settings in such a case would
be a waste of time.
3-4
1. Needle jet 4. Pilot jet
2. Pilot air screw 5. Throttle valve
3. Main jet
6. Jet needle