unstable idle

4 replies to this topic
  • crashtcs

Posted March 29, 2001 - 07:42 PM


I have a 98 YZ400, which I recently bought used so I don't know it's history, and the idle is very strange. It seems to idle good from a cold start, but runs fast when warmed up, and will occaisionaly drop for a brief period (about 30 seconds) then climb back up. I was thinking it is an air leak, but I havn't been noticed any changes to the idle using carb cleaner around the top end. It does seem to be running a little lean though -- I get some backfiring.


  • F-Pilot

Posted March 29, 2001 - 07:49 PM


Check the rubber boot on the hot start button, I had the same symptoms and all it took was pushing the boot back over the lip.

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  • crashtcs

Posted March 29, 2001 - 08:18 PM


Just checked the hot start plunger and the boot was on all the way. The plunger shaft is a little bent, but seems to be operating normal.


  • enmerdeur

Posted March 30, 2001 - 04:16 AM


It may just be running lean in the slow circuit. Open the fuel screw in 1/4 turn increaments until it goes away. If you get past 2 1/2 turns go one size up on the pilot jet and start over. Stock pilot is a 45. I run a 48 in my 99' 400.

99'YZ-400 (Mine)
00' TT-R 125 (Hers)
91' KX-125 (Son #1)
93' CR-80 (Just sold it. Looking for another TTR125 for Son #2)
99' PW-80 (Son #3)

  • jj

Posted March 31, 2001 - 09:27 AM


Definately sounds like lean condition.

Idle hangs = lean

Idle bogs = rich

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