Yamaha YZ125X 2023 Owner's Manual

ENGINE8-1EAM20198ENGINEEAM30590CARBURETOR SETTING• The role of fuel is to cool the engine, and in thecase of a 2-stroke engine, to lubricate the en-gine in addition to power generation. Accord-ingly, if a mixture of air and fuel is too lean,abnormal combustion will occur, and engineseizure may result. If the mixture is too rich,spark plugs will get wet with oil, thus making itimpossible to bring the engine into full play or ifthe worst comes to the worst, the engine maystall.• The richness of the air-fuel mixture required forthe engine will vary with atmospheric condi-tions of the day and therefore, the settings ofthe carburetor must be properly suited to theatmospheric conditions (air pressure, humidityand temperature).• Finally, the rider himself must make a test runand check his vehicle for conditions (pick-up ofengine speed, road surface conditions) and forthe discoloration of the spark plug(s).After taking these into consideration, he mustselect the best possible carburetor settings.TIPIt is advisable to make a note of settings, atmo-spheric conditions, road surface condition, lap-time, etc. so that the memorandum can be usedas a reference useful for future.EAM30591ATMOSPHERIC CONDITIONS ANDCARBURETOR SETTINGSThe air density (i.e., concentration of oxygen inthe air) determines the richness or leanness ofthe air/fuel mixture. Therefore, refer to the abovetable for mixture settings.That is:• Higher temperature expands the air with its re-sultant reduced density.• Higher humidity reduces the amount of oxygenin the air by so much of the water vapor in thesame air.• Lower atmospheric pressure (at a high alti-tude) reduces the density of the air.EAM30592TEST RUNAfter warming up the engine equipped with thestandard type carburetor(s) and spark plug(s),run two or three laps of the circuit and check thesmooth operation of the engine and discolor-ation of spark plug(s).Air temp. HumidityAirpressure(altitude)Mixture SettingHigh High Low(high) Richer LeanerLow Low High(low) Leaner RicherDiscoloration Condition of spark plugNormal Insulator is dry and burntbrown.Over burned (toolean) Insulator is whitish.Oil fouled (toorich) Insulator is sooty and wet.A. NormalB. Over burned (too lean)C. Oil fouled (too rich)ABC