What's the size of the stock pilot jet on a 2001 400ex, and could that be the problem of it coughing and stumbling when wicking it open? Thanks [img]i/expressions/face-icon-small-cool.gif[/img]

 

stock pilot jet is a keihin 38. it is a 21 series if you go to replace it. what mods are done to the bike?

 

Could be...could also be the mixture screw is too lean. Or a vacumn leak in the intake area.

 

.. The mods that have been done to the bike include total ditching of the airbox, replaced with an open element airfilter and a healthy Super Trapp pipe sans discs. Wow-- could a mixture screw do that?

 

After all those mods...the jet is probably too small and the mixture screw wont compensate enough.
The procedure for adjusting the mixture screw, is to start about 2 turns out on a warm motor. Then slowly turn it in, til it drops in rpm. From this point count the number of turns out, til the rpms drop again. Midway is the spot to set it at.
If, however, you get to about 4 turns, ( from seated,) and, the rpm still doesnt drop....the pilot jet is too small.

 

you will need to change your jetting. it may even need like a keihin 155 mainjet. i would also see how it runs by moving the needle clip down one position. also, i would try a 42 pilot jet. the engine builders that i talk too seem to run the fuel screw up too 3 turns out and then get a larger pilot jet