YZ400F help!

1 reply to this topic
  • yz400fdude

Posted May 16, 2007 - 08:55 PM


I had a problem with the way my yz400f was running so first I will tell you what it was doing. First off it wouldnt idle without the choke on no matter what I did to the idle adjustment but it would run with the choke off if I kept on the throttle pretty heavily but it was popping like a popcorn machine so I killed the engine took it back to my garage pulled the carb and cleaned it out then I reinstalled the carb and when I go to start the bike I can kick it over without using the compression release and that was never possible. So my first thought was that there was something wrong with the compression release valve but it seems to be working fine. Does anyone know what would be causing this? If its something wierd you will have to explain it to me im not a 4-stroke mechanic. Thanks alot.

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  • grayracer513

Posted May 16, 2007 - 10:20 PM


I know you said you cleaned it out, but it sounds from the symptoms like your pilot (idle) circuit is obstructed somewhere along the line. Remove the pilot jets and the air jets at the carb mouth and run a fine wire or nylon bristle through them to insure that there is not some varnish in or across the orifice. Blow compressed air through each passage. Verify that each one runs through to its end by spraying carb cleaner through with the "straw" nozzle.

I have also seen that this sort of problem can be caused by a defective or improperly adjusted Throttle Position Sensor. The bike will run reasonably well in a default mode if you disconnect the TPS, so you might want to try that and see if it clears up.

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