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1999 yz400 idle won't come down


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please help, i live in southern ca., i ride in the desert alot and my 1999 yz400 has a fmf tip on it other than that its stock, i have taken the air box lid off also, my bike has ran fine for the last 2yrs but within the last couple rides the idle won't come down after it gets hot, i have messed with the adjuster but it makes no differance, i have also cleaned the carb several times, any ideas ??

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If you're positive the carb is clean (did you take it apart and blow everything out?) then my next best guess would be either throttle cable adjustment or routing (make sure there is a tiny bit of slack when you back off the throttle completely) or an air leak somewhere between the motor and the carb. Also if you haven't yet check the carb side ends of the throttle cable to see if it's frayed which may cause it to bind.

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My guess would be if it starts and runs good, it's probably the pilot air jet. I would take the carb apart one more time and make certain that everything is cleaned well (pay close attention to the pilot air jet) and go from there.

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I think you have gotten some good troubleshooting info so far. I will add that to ensure your throttle cables aren't the culprit, loosen them up all the way in the brackets on the carb. If the cables aren't adjusted correctly when they are tightened it will cause this to happen. Then start it and see if the bike idles. Also the cables fray over time and it may be hangin' up on a bad cable so look for that also. To check for an air leak, with the bike running spray some starting fluid around the carb boot and see if the idle goes up. If so, you have just pin pointed an air leak. This can be dangerous and do it when the bike isn't real hot and don't spray towards the exhaust pipe.

Good luck,

Pat

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If none of the earlier suggestions do not remedy your issue, you may have a cracked throttle slide plate. This plate is inside your carb, and it is prone to cracking. My WZ400 has injested 2 so far.

Just something else that may be causing the lean condition.

Brandon

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This is not a hard one to remedy. The bike is running lean for sure. If the bike has been running good and then all of a sudden, hold's the RPM's high, the most likely curprit is an air leak. The diagnosis can be confirmed by raising the idle sufficiently to keep the bike running and spray some WD-40 around all the air openings (1 at a time) starting at the Head side of the boot....If the rpm's change when an area is sprayed, that will be your culprit. If it does not change at all, then first pull the carb and inspect the slide for cracks...(additionally on the 98/99's only...Open up the accellerator pump and make sure that the diaphram does not have a hole in it....This has been known to cause this problem as well) If none are found, adjust the fuel screw out to two full turns....If the RPM's do not drop still...try raising the pilot jet 1 step up...But you should not get that far.

More than likely your going to find a carb boot not seated properly or mabey a tear in the pump diaphram.

Bonzai 🙂

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