Engine acting up '98 YZ400F

Brand Yamaha Yamaha YZ400F 1998

4 replies to this topic
  • bjm904

Posted May 19, 2014 - 06:13 PM


I have a 1998 Yamaha YZ 400. It has run good since I have owned it. At the end of last season the engine started acting up. It has no power at all. It idles good but when any load is put on the engine It starts misfiring and backfiring and will barely move. I have checked the spark plug for fire, checked all the jets and carb, all clean and seem to be operating normally. I have drained the oil and there is no metal filings in it. Prior to this issue, the engine was running hot and would overheat. Thank you in advance.

  • bjm904

Posted May 20, 2014 - 07:43 PM


any help?

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  • Slow and Jerky

Posted May 21, 2014 - 04:09 AM


My first thing would be to put fresh gas in it (make sure to drain the bowl), check the exhaust make sure you don't have some sort of restriction. Any time I hear that something will idle but no go I think about the old potato in the muffler trick from high school  :devil: Dirty jets or maybe an accelerator pump gone south are also likely candidates.

  • Yz400f Jeff

Posted May 21, 2014 - 12:32 PM


Sounds like the problem I had when I forgot to put my accelerator pump rod in... Maybe yours is plugged up.

  • grayracer513

Posted May 21, 2014 - 01:33 PM


You say it started this before it was stored?  Check the vacuum release plate on the carb slide for cracks or missing pieces from the corners.

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