Question about Yamaha YZ 250 Motorcycles
There should be a small air gap at the bottom between the slide and the venturi. A couple things to check are (we'll start easy and progress from there):
Remove the carb from the air box and the intake boot, but do not remove the top screw cap or cable from the carb. Once you are holding the carb in your hand, twist the throttle and see if the slide moves up and down freely. Does it return to the botton of the venturi on it's own when the throttle is released (or you twist the throttle forward)? If not, and the slide is sticking open, it is allowing more fuel/air in which is the problem. There is a spring under the top cap which should push the slide back down when you release the throttle. If it's sticking open, it's either a broken cable (you wouldn't see the slide go up and down when you twist the throttle if it's broken), or an unlubricated cable, a piece of debris in the carb between the slide and the carb body, or the spring is whacked. Unscrew the top cap, slowly pull the cap, spring, and slide up and check the spring. You can't miss it, it's a big one right under the cap. If it looks ok, check for debris on the side of the slide and in the carb body to see if there is sand/dirt causing the slide to stick open. If all of this checks out ok, then check the cable for free movement when you twist the throttle. If it's sticky, lube the cable. If it's ok, move to the next step.
On the bottom of the slide you will see the jet needle. It's long, skinny and tapers at the bottom. Try moving the needle up and down. It should be pretty snug. If the "c" clip that holds the needle came off, it will sometimes allow the needle to remain open which will cause this problem too. If the needle is ok (in place nice and solid and doesn't move up and down freely), then go to the next step:
Remove the float bowl and check to see if the float, float valve (under the back edge of the float where the float attaches), and seat (where the valve rests in the carb body) are all in good shape. If the float has a hole in it, or the tang holding the valve clip is bent, it will cause the float to drop down lower in the bowl, which opens the valve and allows more fuel in. Also check that the clip that connects the valve to the float did not come loose. If there is a problem with the float or float valve allowing too much fuel into the bowl, the excess fuel runs into the engine (via vacuum when the engine is running) and will cause the problem you are seeing. In this instance though, there is normally excess fuel coming out of the overflow hole on the side of the carb when the bike is not running and the fuel is on. If your problem just happened once and you shut the fuel off (and haven't turned it back on yet), put the float bowl back on, reinstall the carb and try opening the petcock (gas shut off valve). Let it sit for a few hours or overnight with the gas on. If the float or float valve is bad, you should see gas drip out of the overflow on the side of the carb. You may not have seen this if the problem just started happening while the engine was running. It was sucking the excess fuel into the engine causing it to rev wide open. When you shut off the gas flow, it prevented having the excess fuel drip out the overflow hole and letting you know it was a float problem. Hope this helps.
Posted on Nov 19, 2009
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Posted on Jan 02, 2017
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