please forgive if i sound less than intelligent in this. we own 6 bikes from trx 450er to 250 . currently getting butt kicked by 2001 trx 400 ex.a new machine to us. i believe the bike is jetted, definitely piped, and k&n. when you mash on the throttle it has horrible lag. it has no where near the characteristics of its twin that runs strong from 800 rpm to redline. i suspect carb incorrect jetting or too much air too soon. if that isnt enough... i went to pull the plug ( which looked good sand beige in color) and now it acts llike its cross threaded going back in. what a day , all this after 10 hrs of work, doing a complete install of a 300 ex (victim of chain "case break"). so now im more concerned of how to chase threds, and then get back to the original problem to solve, huge throttle lack and just seemingly wierd acceleration. like the power curve is short and crummy after a huge lag of the line. god i hope im writing this in the right section. i dont think im retarded, but after the day ive had im pretty discouraged.

 

If I'm understanding you right it might be as simple as the throttle cable needing adjusted. Or if the bike revs high but its really not going that fast it might be a wore out clutch. Also you might want to check the air box lid and carb to make sure they are tight and there are no air leaks.

 

I would suspect jetting also. You said it "might" be jetted and had a pipe. First thing I'd do is check to see what size jet is in it. Might need to be bigger maybe smaller. Either could cause that problem. If it's jetted to spec with the airbox lid off and the lid is on you'll have similar probs.

 

Not only check the main jet size but check where the clip is on the needle, if it's older than a 2005 that is. 2005 and newer 400ex's don't have adjustable needles.

 

Were you using the supplied plug wrench to remove and replace the plug?
It is nearly impossible to remove or install the plug in the 400ex without the stock plug wrench.
If you did cross thread it I would recommend you remove the head and have it helicoiled.