lean YZ400?

4 replies to this topic
  • Dafan

Posted August 27, 2004 - 10:10 AM


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

  • Dafan

Posted August 27, 2004 - 10:13 AM


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

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • TonyWilliamsX

Posted August 27, 2004 - 10:21 AM


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?

  • Dafan

Posted August 27, 2004 - 03:13 PM


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...

  • Butta

Posted August 27, 2004 - 11:07 PM


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?

Related Content


yz250x vs 450 4t comparisons (preferably desert riding) needed by JakeNorthrupYZ450F

Dirt Bike   Make / Model Specific   Yamaha   Yamaha 2-Stroke
  • 9 replies

Is it worth it? (Cam and High compression piston) by macgi77

Dirt Bike   Make / Model Specific   Honda   CRF 150/230 F/L
  • Hot  36 replies

Help with 2000 yz426 by weekend woods racer

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • Hot  36 replies

Thinking about Yamaha 250... by Arctic Pride

Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  491 replies

Chubby dad, looking at bikes , First trip to the orange/black forum ! by Slow_ride

Dirt Bike   Make / Model Specific   KTM   250-530 EXC/MXC/SXC/XC-W/XCR-W (4-Strokes)
  • Hot  28 replies

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.