Need Help Inconsisting Idling

Hey Guys,

I'm a fairly new owner of a 99 YZ400f and don't know too much about them. My bike cold will only start without the choke. It only takes a couple of kicks to start without the choke. If i use the choke it must flood and it takes me for ever to start it. It idles fine when it's cold or semi warm, but once it's been ridden for a little while, when i go to stop it just stalls. It starts up fine again but continues to stall if i dont keep it revving a little. I turn up the idle in an attempt to keep it from stalling, but then when i stop it idles really high for while and then slows down again. The idle continues to fluctuate up and down like this until i stop and give it a rest. During this time the bike still starts up fine. I do have a problem where it uses excessive amounts of coolant whilst riding (probably head gasket), i was wondering if this would affect the idling. Does anyone else's bike start without the choke when it's cold, i've have never heard of this before. I'm unsure of the jetting in my carbie. I was wondering if i have a head gasket problem & carbie problem or is the overheating issue affecting the idling. The bike feels fine when im on the gas, it only plays up when it idles.

Appreciate Any Help or Advice


Check the pilot/idle circuit. Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method.

Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.

*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***

Turn the screw in until the idle becomes rough or the bike stalls.

if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.

If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.

Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.

If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.

If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.

If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,500 rpm, but go by the spec in your manual.

If your bike will start and run well when cold without a choke, it's way to rich for some reason, which is why it won't idle once warmed up, most likely. The cause could be a number of different things, but the most common are incorrect float level, and the idle mixture set wrong, although it's a little hard to picture the idle being that rich just because of the setting of the fuel screw.

Since you bought it used, and assuming it's been like this since you got it, then the list of possibilities expands to include something that the previous owner did, like put in the wrong jets, etc.

Did you get a manual with it? If not, try downloading one here:

The YZ400 file was corrupted and would not open last time I looked, but it may have been fixed. You can use the manual for a 2000 WR400, or a 2000 YZ426, although the carb is a bit different in the 426.

By fuel screw do mean the pilot screw, there is no fuel screw listed in the manual.

By fuel screw do mean the pilot screw, there is no fuel screw listed in the manual.
Yes. We refer to it as a fuel screw for clarity, since pilot screws generally meter air. The fact that they meter fuel makes them work "backward", as in out for rich and in for lean.

Thanks i just completed the pilot circuit check, and the bike still runs with the fuel screw closed. It also starts with the screw closed. I guess i need a smaller pilot jet. I'm not sure which one is in my carb, will have to check it out.


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