I have a 04 TRX400EX that i had recently gotten off of my buddy. I knew there was an issue with it, but i took it out this past weekend riding and it showed no problems at all other than the typical blue smoke. After riding for about an hour- hour and half, i shut it off to take a break. Went and started it back up, rough to start and i could rev it up to a certain point slowly advancing the throttle but if i tried to press the throttle hard and quick, it would pretty much die but i kept it going. Need some help on this. I have worked around plenty of fourwheelers that ive owned but never have had a problem with a 400ex like this one. Please let me know if you have any pointers! Thanks!!

 

Give it a full service, plug change, set valve clearances etc. If that doesn't fix it, check the carb over, clean out jets, check choke works properly and check diaphragm (if fitted) for holes. The EX is a bike that owners "tinker" with, so wrong jets for the state of tune are not uncommon, but if the plug colour looks OK and you are still having problems after checking the above, it could be an electronic fault, coil or CDI breaking down when hot.