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2011 kx250f problem. Need help asap

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My brother had his 2011 kx250f rebuilt about a month ago.  We bought the bike blown up and it had been setting outside for 6 months, some people. Smh. But anyway. We put a new crank, piston and cylinder on it. We used a wrench rabbit kit.  The bike ran great for the last 3 or 4 trips out. Today we went to ride and second practice moto the bike started acting up. It starts first kick and will rev out in neutral but when you put it in gear it pulls good and hard till about 1/2 to 3/4 throttle and it starts sputtering very badly. Let off and slow down then it will do it when you get back up in the rpms. Bike has a new plug when we rebuilt the motor and we put gas in it today on the way to the track. I also filled my 2012 crf250 up at the same time and its fine. So i dont think its fuel. Any ideas?

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Hey Racer94.  Sounds to me like the valves are seating in nicely and it is time to go in to check and adjust(shim) the valve clearances.  Start there.  While you are in there and before you remove the cam chain, check that the timing marks and the cam lobes are all in the right places - in other words confirm that the timing is correct.  There is a slight possibility that a cam sprocket is slipped on the shaft a bit causing timing to be out (even though the dots line up).  This can cause the 1/2 - 3/4 fade you describe.

So, go in - verify timing, and then check and adjust the valve clearances.

You will be out ripping' up the track again quickly.  :ride:

 

Edited by FaceDeAce

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37 minutes ago, FaceDeAce said:

Hey Racer94.  Sounds to me like the valves are seating in nicely and it is time to go in to check and adjust(shim) the valve clearances.  Start there.  While you are in there and before you remove the cam chain, check that the timing marks and the cam lobes are all in the right places - in other words confirm that the timing is correct.  There is a slight possibility that a cam sprocket is slipped on the shaft a bit causing timing to be out (even though the dots line up).  This can cause the 1/2 - 3/4 fade you describe.

So, go in - verify timing, and then check and adjust the valve clearances.

You will be out ripping' up the track again quickly.  :ride:

 

thanks, i may try that.  my only concern is that the bike starts first kick and will rev to the rev limiter in neutral. its only under a load in gear an wide open throttle. we didn't put new valves in an they was adjusted when put together. wouldn't it being out if adjustment or time make it start hard and not rev out in neutral? 

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1 minute ago, RM Racer94 said:

thanks, i may try that.  my only concern is that the bike starts first kick and will rev to the rev limiter in neutral. its only under a load in gear an wide open throttle. we didn't put new valves in an they was adjusted when put together. wouldn't it being out if adjustment or time make it start hard and not rev out in neutral? 

it also feels as though its losing spark when it does it.

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well. kawasaki scanned the bike.  no trouble codes.  fuel pressure test shows good.  new plug new gas.  cleaned the injector.  bikes in time.  valves are good.  any ideas?

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