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Dafan

lean YZ400?

5 posts in this topic

I bought a YZ400. It didn't start very well so I checked the carb.

float height 9mm, stock jets. everything clean, no air leak. It now starts easier, but won't idle very long. With the idle all the way set high, it will idle for about 20 seconds then sputter and die. Rev used to hang, but I backed pilot screw out to max (in increments) and now it doesn't hang but still back fire coming off throttle. And still dies when idling. will idle fine with the choke pulled out a little.

any ideas? def. lean? should I lift the needle to see what happens? I don't have any jets to play with right now and going to ride tomorrow. It is ridable but I would hate to stall a lot because it is such a pain to start, I am short and small...

any help appreciated

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also its got stock headers and pipe. But it sound pretty loud, maybe the previous owner freed it up a bit

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My '98 was quite lean, proven most convincingly when I rode it at 7000ft elevation Mammoth Mountain, and it ran fine with stock jetting.

Jetting issues need to first take into consideration the "density altitude". That is basically elevation, heat and humidity. There are ways to figure that out (airplanes do this).

So, first, how high, hot and humid is it where you ride?

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well this baffles me. I am at about 10ft-100 ft above sea level, it is 85 today and not too humid (philadelphia). it should be rich by all thinking...

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Although I don't have a lot of experience playing with jetting, it seems perfectly clear that if it idles fine with the choke slightly out, then it's gotta be lean. Did this bike run great before? Did it run great in the winter? Stock jets, stock pipes, etc....doesn't necessarily mean that it is jetted perfectly. What's the spark plug look like?

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