unstable idle

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.



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.

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.



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)

Definately sounds like lean condition.

Idle hangs = lean

Idle bogs = rich

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