Question about 2004 Yamaha YZ 250 F

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MY YZ250 WONT REV PAST 3/8 THROTTLE. I NEED HELP!!

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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Jetting on 1994 yz 250. the main jet is a 330 the pilot is a 40 and the needle is a 61. we mix the oil to 50:1 but it loads up onan idle and in the middle. we are at 5000' elevation.


Hi, Anonymous it should be noted that your performance issues are not electrical ie. faulty charging system, battery condition, and connections etc. If your fuel system (gas tank, filters, fuel valve and carburetor) is contaminated with ethanol sludge, varnish, rust, dirt, water etc. or your bike has been sitting for months or years without running these components must be "PROPERLY" cleaned and reassembled "CORRECTLY" before any adjustments can be made. Tuning your carburetor is fairly simple once you understand the basic principals. You engine is a simple airbox sucking air in and blowing it out, it is finely tuned at the factory for maximum performance once you upset that delicate balance by changing air filters, camshafts or exhaust systems your performance may go down the and the engine may run poorly, you need to compensate the air-fuel mixture in the carburetor in order for the engine to run smoothly and at peak performance. If you are running multi carburetors you need to sync them first and make sure your air cleaner element is clean and dry for paper elements or lightly oiled for foam and meshed elements and properly installed. Here is how and where you compensate trouble: "TIP" if your engine "BOGS" your not getting enough fuel.
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1. Closed to 1/8 throttle is managed by the air screw and pilot/slow jet.
2. 1/8 to 1/4 throttle is managed by the air-screw, pilot/slow jet, and throttle slide.
3. 1/4 to 1/2 throttle is managed by the throttle slide and jet needle.
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5. 3/4 to wide open throttle is managed by the main jet and air jet.
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1 Answer

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.


Spitting oil indicates that it is running rich. It could be down on compression which will cause it to run rich. The rings don't last forever in these bikes and need to be changed every once in a while to restore proper operation.

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.

Good luck!

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1 Answer

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Jan 03, 2010 | 2003 Yamaha YZ 250

2 Answers

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.

Jan 19, 2009 | 2004 Yamaha YZ 250 F

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