Question about 2003 Yamaha YZ 250 F

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2002 YZ250F wont start

I changed the air filter, cleaned the carb and changed the plug. Still wont start. I've been adjusting the idle but have no idea if i'm doing it correctly. Help.

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Assuming all other things are working properly(carb and ignition), check the valve clearances. Specifically look for any valve that doesn't have any clearance. This is very common, especially with bikes that have alot of time on them. If so, have the valve(s) adjusted, but keep an eye on any that had closed up. If they close up again in the near future, you'll have to have the valve replaced.

When the valves do this, they are held slightly open all the time and let your compression bleed off, causing it to be near impossible to start. If the bike does start, it will be much easier to start again once it's warmed up. But back to the same when it cools off.


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Posted on Jun 12, 2010

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Make sure your spark plug is not fouled and the gap is correct, also air filter is properly cleaned. If it still wont idle, locate the idle adjustment on the carb. There are 2 screw type adjustments there, one is the idle/air screw and the other is idle adjustment which is the one you want. Start the bike and warm.it up, slowly turn the idle adj. Inward like tightening a bolt, periodically give the engine a slight rev and let it idle.down. Do this until you reach the desired rpm you want to idle at and your finished

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This is my occuring problem I've been having: Bike starts up fine and will run fine in any gear if going slow and not opening the throttle lots. But as soon as open throttle lots the bike instantly...


this does is not related to the idle adjustment. It sounds strange that there is no fuel filter, there should be. the screen is for only stopping any large debri from going to the carbs. follow the fuel line from the tank valve and I will bet you will find a fuel filter. i also would suggest draining the fuel bowls on the carbs and catch the fuel. check for any debri in this gas. You may try running some seafoam carb cleaner thru the carbs. After changing filter and bowl draining does not work you may have to remove the carbs to clean them and make sure all is operating correctly ( slides). be sure that your air filter is also clean.

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Time for carb cleaning, blow the jets with compressed air and check the fuel level in the float chamber, adjust if necessary.
Tht factory plug is where the air-mixture screw is and tht plug has been kept there for non tampering!
That screw is factory set.
Basically its the primary jet which is a problem in your bike's carb, but all in all might as well blow both of them.
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I have a 2002 yz250 that wont idle. It will start everytime and run as long as your on the gas but dies when you let off the gas.


Drain the carburetor. There should be a screw on the lower side or a plug on the bottom of the carb float bowl. Remove the screw/plug then replace it after the fuel drains. Remove the carburetor from the engine.

Remove the float bowl and clean the entire carb with a spray carb cleaner from the auto parts store. Wear protective goggles to avoid getting spray in your eyes. Spray into all the little airways and fittings in the carb. Remove the idle screw and the air screw on the outside throat of the
carb and spray into the screw holes as well. The diagram below depicts the problem area. The bike will idle properly once the pilot jet and related passageways are cleared.
< < READ CLOSELY > >
Be sure to put these two screws back in the same hole they came out of. IMPORTANT > do not tighten these two adjusters down. Only screw these in until they LIGHTLY seat. Now turn each adjuster one and one half turns outward. Put the
carb back together, clean the air filter and install the carb. Let the float bowl fill then start the engine.
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