Yz400f running really bad

Hi Micke here and im all fresh here and not so good to write in english so bare with me:)

Ok i have a yz400f 1999 that is running really bad. I have searched the forums and found a Lot of threads about almost the same issue that i Have.

The top end is rebuilt so the valve clearence is good.

My bike starts but it wont rev up properly.

The engine reaches a high temp really fast.

It pops when i reduce the throttle.

Lack of power.

This happens almost every time i start it after a couple of days. But sometimes i leave it for a couple of hours and try to start it again it runs good.

I have cleaned the carburator and replaced some worn parts, o-rings, needle valve, acc pump rubber.

Anyone got any ideas?

Btw i have a new needle valve seat with size 3,6.

The one thats in the carb is size 3,8. Will it not work properly IF i change to the 3,6? I mean its only a 0,2 difference beetween them?

It sounds like your stator is going bad

Do a stator test per the service manual

Did it run like this before you cleaned the carb?  A common mistake when reassembling is to install the vacuum release plate onto the engine side of the slide upside down.  One end is slightly rounded, and looks like it's mad to be one the bottom, but it need to be on top.  The "square" end with the hole near the edge is supposed to be on the the bottom.  Could also be water or dirt in the fuel.  Remove the plug on the carb bowel and have a look.


Running "OK" after sitting undisturbed for a time does raise the possibility of a stator malfunction.  Certain conditions within the coils can make them sensitive to heat, and they may only act up after they get warm.  Because of that, they may test fine if tested cold.  Even when warm, the fault may not show up so easily on a meter, but you can sometimes get the meter to show a change if you tap on the case cover while watching it.


Service manuals are available here: 





I have measuered the stator with the following results and according to what the service manual says im quite satisfied even when the temperature wasent fully 20 degres celcius as the manual recomended.

Pick up coil W - R 292

Suorce coil Br - G 880

Suorce coil 2 P - B 55.8

My numbers on the ignition coil is making me confused atm so i might be on someting here!

I ran it today and it was going great for like 20 minutes untill it started to run wierd again.

At WOT it sounded like i was on the rev limiter but i wasent anywhere near it... After that it ran good again and all the suddenly it was going crap again, popping, lack of power. After that i waited a couple of hours and made the measures on the stator.

Check to see if the light blue wire running between your neutral switch and the CDI has a bare spot that may be shorting out part of the time.  

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