Question about 2004 Yamaha YZ 250 F

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My 96 yz-250 will run fine in lower rpm's but once you go past 3/8 throttle it acts like its bouncing of a rev limiter. It bounces back and forth really quike and wont accelerate anymore.....could this be my cdi box going bad?

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It does sound like an ignition timing problem, but could also be a breathing issue, air flow in or exhaust flow out could be restricted i.e.:dirty air filter, excessive carbon build up in exhaust pipe from burning too much oil or rich fuel mixture.
possibly and more likely some junk got into your fuel system and partially plugged up the main jet in the carb, -or has the needle on the slide been moved? The little circlip at the top of the needle, if it has one.
1.Replace the spark plug, check gap, check spark, it should be bright blue and fat.
2. check the fuel filter, if no filter check the carb for dirt.

Posted on Jan 20, 2009

  • irondog06 Aug 30, 2011

    I had an older YZ and it was doing the same thing,on my bike one of the the wires on the stator was broken not allowing the engine to advance.The bike would start up nice but would just not rev up.


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I have a 94 yz250 has plenty of power but is spitting oil and after about 20 min of riding goes dead but will crank back up right after. doesnt do it if I ajust the carb to run rich.

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I am betting you are turning the air screw to "adjust" it to run rich as you stated. This screw only effects the lower RPM - it is actually a fine tuning screw to adjust the low end - specifically the pilot and needle jet. It has very limited effect on the jetting above maybe 1/2 throttle. The proper way to alter the way the carb acts is to physically change the jets in the carb. The pilot is for lower end performance, the needle is for the lower-mid range and the main is for mid to mostly upper RPM range. The float level also plays into all this.

To attempt to answer your problem - it could be that your spark plug is incapable of firing once it gets overheated. An overly rich mix will prevent it from doing it's job also and can burn a plug out. Try replacing it with a fresh plug and you might want to clean your air filter and check your compression. You should have at least 150PSI. I've seen bikes run with as little as 95PSI but they run poorly and are hard to start.

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Wont rev past 3/8 throttle!! I NEED HELP.

could be. I had a 91 kx 250 that was acting crazy and it turned out to be the stator and cdi box. you can check the stator easly with a omes meter but the cdi box is nearly impossible to check. you pretty much have to buy a new one and try it. also check to see if your power valve is working.

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