Question about 2003 Yamaha YZ 250

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Only runs half throttle

The engine only runs half throttle smokes blue and sometimes black it has a new carb if u let off the gas it dies immediatly

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  • Sharky Feb 07, 2012

    It sounds like your fuel ratio is out, too much oil. What mixture are you using? My guess is your plug is fouling & at idle the spark is too weak to run. Use a fully synthetic oil at 50:1. Also check your float level.

  • Sharky Feb 07, 2012

    Also it could be nothing to do with fuel. Who diagnosed the carby as the original issue? It could be a problem with your ignition. Weak spark can cause these symptoms too as it's not burning the fuel correctly.

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So, can you tell me what kind of carb was on the bike before? was it factory? and what kind of carb was just installed? blue smoke can be caused by burning transmission oil or a rich fuel mixture. that might be a bad crank seal or case gasket, or just improper fuel/oil ratio or improper jetting. black smoke on the other hand is usually caused by too much gasoline burning, but if there were enough gas entering the engine to cause black smoke, the bike probably wouldnt run at all. but, if you can give me more info on the carb swap, and if possible any jetting information or jetting changes, thenI can definitely tell you what your problem is.

Posted on Sep 04, 2009

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Go Cart. Starts and runs only on full choke. 4 cycle, gas. Open choke, and engine dies. Sat for over a year. New gas, throttle working, spark ok. Can't find any air leak; carb never off of engine.


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1 Answer

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I would suggest starting with the less expensive solutions first. If you pull the air intake hose off the throttle body and open the throttle with the engine off look down inside if it is black try spraying it out with throttle body cleaner make sure to spray it out good. Afterwards start it you may have to keep your foot on the gas for a few minutes to make sure it doesn't stall then take your foot off the gas pedal and see if it stays running if it does your throttle body just needed cleaned. Note do not use carb cleaner use throttle body cleaner only, carb cleaner can damage the sensor in your throttle body.

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87 325 runs good when


It's the automatic choke!

The choke is "closed" or barely open... When cold, the engine needs a RICH mixture (more gasoline, less air). When the engine warms up, it requires a LEAN mixture (less gas, more air).

It is probably creating BLACK tailpipe smoke ... that means unburnt gasoline. (Blue smoke is OIL, white smoke is WATER).

Put the parking / emergency brake ON.
1. Remove air cleaner assembly from intake, use a carb cleaner spray or throttle body spray cleaner and spray just a little. (AutoZone $5)
2. Find the accelerator2.gif arm under the hood.
3. start car, rev the engine to make sure how to operate the accelerator from under the hood.
4. Carefully spray into carb / throttle body and work the accelerator ... don't let the engine die...
5. Watch out for "backfire"... it may create a small flame out of the carb. It will extinguish itself in one second.

6. You may need to do this two, three times.
7. Add a "complete fuel system2.gif cleaner" to your next tank of gas ($8).

That's it! ... pretty cheap, too.

Hope this helps!

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ck for water in carb bowl. also ck for filter in gas tank, or something in gas tank.

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Car dies at idle


I think a top end cleaning would be of value to you.

1. Remove inlet hose and clean with carb spray
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3. With engine running, open throttle (wide open throttle) and hold hand over inlet and then take your hand away and allow it to flow again and release the throttle. Don't over rev it, you just want to create a vacuum state in the intake that will remove carbons, etc. Do this open throttle and cork it with your hand about 6-8 times. Then reassemble anything you took apart.

This trick removes stubborn gunk in the intake by loosening it with carb spray first and creating a vacuum to dislodge it. Expect a little black smoke when doing this trick.

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