HELP with my YZ400

I have a 1998 YZ 400 that I am having some issues with. It has a problem idling and a hiccup right off the bottom when you first start to open the throttle. I have been told it could be jetting, valves, accelerator pump, etc. I wanted to get some feedback. Please let me know what you guys think. It is driving me nuts. Also the idle adjust is all the way up and it will barely idle and usually just die. I ride it on the street and have to keep revving it at the light so it will stay running. It runs fine but idles high with the choke on. It has a new motor, and the only thing I know is that the main jet is a 175 and it has a DSP pipe on it. Thanks in advance.

For jetting help we'll need to know what elevation you're at as well. I'm at sea level, and a 175 (I think) was what I had in it with the stock pipe (could have been a 172, though). How long has it been since you cleaned the carb??

Thanks Butta. I ride at sea level also. I cleaned the carb tonight. I have also tried running with the stock pipe and the problem is the same.

To me it sounds like your pilot jet is pluged. Your bike needs the idle circuit to run. When you keep reving it up at the light your not on the idle circuit anymore, there for it will run. It will idle high with the choke on that is like the same as opening your throttle to give the motor more air/fuel mix. With the choke on its just giving the motor more fuel to burn so it will idle higher. The 175 main jet has nothing to do with your idle at all. The main jet only effects 3/4 to full throttle. On most mx bikes you can get away with just changing the needle position unless your running it for along ways on WOT.


for your hicuup, check your accelerator pump chamber and actuator rod. they tend to get water under the boot and rust up and stop working.

Thanks guys. I will check out the pilot jet. The accellerator pump was completely rusted up. I appreciate all your help.

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