Cleaned carb ,changed fuel filter . is it diaphragm in fuel pump, seems ok, not clogged
Hi There, Assuming we are talking about a Yamaha YZ 250 (not sure of year, but it should not matter in this case), it sounds like you may have a fuel starvation problem at high engine speeds. The high speed load screw (also sometimes called the power adjusting needle or main adjusting needle) controls fuel flow at high speed or under full load. This should be adjusted only when engine at normal operating temperature to avoid changes in how the fuel is vaporized at various temperature changes (i.e. fuel atomizes differently when cold vs hot). If this is not properly adjusted, your motor is essentially starving for fuel as you twist the throttle. I would recommend taking this to your local motorcycle shop to have this properly tuned as this can be a bit tricky on a two stroke engine which can result in a lean motor. This type of condition can eventually cause the motor to seize up.
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