I have a 2004 DVX 400.
If I have it in neutral, it will rev smoothly.
If it is in gear, even with the clutch held in it sputters and pops anywhere past 1/4 throttle.
It will randomly run fine, but that's few and far between.
I have replaced, battery, stator, coil, plug, and solenoid so far.
PLEASE ADVISE!

 

Have you checked and cleaned the carb?

 

I have not had it apart yet.
I am starting to wonder if the CDI might have issues. It acts like the Rev limit is stuck very very low. It never gets past a certain point power wise. It just pops steadily till I back off.

 

Maybe give a try.