high idle with backfire

8 replies to this topic
  • blmyachtsales

Posted December 05, 2008 - 07:05 PM


Finally joined TT after several weeks of reading posts. Trying to help a friend here with a 1999 YZ400. Bike idles high for a long time before fianlly comin down then it stall easilly, pipe gets red hot real quick, plug is white (all tell me it's lean) but it back fires like it's running ritch and if you pull the choke while hot it dies like it's flooding out. Need some ideas, pilot jet is a stock 65, turning fuel screw out doesn't help, turning it in didn't do much either (didn't kill it), cleaned the starting, pilot and main jets as well as the corresponding air jets and passages, clean air filter, new plug. Oh it also has a pretty bad flat spot just off idle, cleaned a badly mudded up accelerator pump and this made it better but still very noticable. One thought, could a bad coil give any of these symptoms? Thanks and great to be here.

  • blmyachtsales

Posted December 06, 2008 - 05:22 AM


Would a vacume leak also cause these symptoms?

  • redlineblues

Posted December 06, 2008 - 08:32 AM


I would have to agree with blmyachtsales sound like one of the rubber boots on the carb leaking or not enough fuel. My 426 had the same symptoms when the petcock was partially plugged.

  • jrny426

Posted December 06, 2008 - 09:23 AM


check your carb mounts (clamps) and exhaust for tightness leaks at both end can cause these problems,my 426 also had the same thing

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  • blmyachtsales

Posted December 07, 2008 - 05:43 AM


Thanks for the tips guys, checked the carb clamps and cold start circuit (did the old spray starting fluid around them to look for a leak) and exhaust clamps and they are fine . Further cleaning and adjustment has helped but the bike still stalls pretty easy in the woods and any qucik smack of the throttle form idle will stall it (maybe I am just used to my kdx220), Back firing is present but only after real hard throttle which I think is acceptable. We did add a 13T front sprocket but still not great. The slightest tap of the throttle at the wrong moment at low RPM and it dies. Also tried limiting AP travel, checked, valve clearance, all basic carb adjustments are stock except the p/J is a 48 instead of a 45 (we are at sea level and 45 Deg.). plug looks good now, the coil did test .018 ohms and was jumpy istead of .020-.030 also the secondary coil test was just off spec (can't remember exact numbers).

  • qwik12beat

Posted December 07, 2008 - 08:51 AM


If the bike doesn't stall when you turn the fuel screw all the way in. You need a smaller pilot

  • motojoe_23

Posted December 07, 2008 - 06:23 PM


Sounds like a lean hanging idle. If you feel the jetting is close, then adjust the mixture screw out a 1/4 turn at a time until the idle drops properly. May have to adjust the idle as well during this.

  • motojoe_23

Posted December 07, 2008 - 06:25 PM


AS to the throttle stall, have you checked the AP diaphragm? it may have a crack in it. The AP is what "fixes" off idle bog. It squirts fuel when the intake air stalls due to a whack of the throttle. So I would check it out more for that problem

  • BC3

Posted December 07, 2008 - 07:55 PM


[COLOR="Blue"]Tight exhaust valves can cause your symptoms.....Check the clearance before you start changing jets and screwing with the carburetor.
Valve clearances change...jets don't...So if it ran good with that jet sequence
once...it still should now.....Think about it.. Make sure a back fire didn't pop the carb out of its manifold (air leak) [/COLOR]

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