Hi I have had my 400ex over a year and just installed a 440 big bore kit on it and I got about 3 hours on it and it started sputtering and sputtering an cutting out so we clean the carb out real good didnt help tuned the carb still didnt work changed the spark plug on it an still didnt do the trick I ajusted the valves as well and now it will only run when I hold the throttle down over half way and it smokes a lot and not sure what to do next any ideas???

 

I'd start by checking compression.

 

Hmm. Jest stating the simple, did you do the big boar right? Maybe you got the wrong sized jugs/pistons, or installed them wrong.

 

it ran fine for about 3 weeks