Question about 2003 Yamaha YZ 250

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Backfiring my bike keeps backfiring n wont start av changed spark plug checked the carb am out ideas now its yz250

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Check the chassis wiring whe it enters the side cover on the ignition side of the engine. There is supposed to be a rubber grommet there. Soetimes that grommet gets kicked loose where is comes out of the side cover hole and the wires chafe to the point that they ground out the ignition module.

Also looks for bare wires under the gas tank around the ignition coil module. Everything your describing points to a chassis short to ground.

Also the problem can be in the handle grip with the kill switch or a bad kill switch.

It is probably time to break out the Ohm meter and start chasing wires.

Kelly

Posted on May 04, 2010

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Check your plugs. If all four are sparking then your carbs are out of sync. They will need to be re-balanced.

If you are capabable in doing this job your self, you will need a screwdriver that fits your adjuster screws and a CarbTune Carb balancer.
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To start, unscrew the adjusters all the way out. then screw them in until the 2 outer carbs move at the same time as the inner 2 carbs
Once this is done then adjust the centre balancer to syncronise the 2 left hand carbs to the 2 right hand carbs.
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