i own a mechanic shop in margate, just bring it here and ill do it for you.

I had a 400EX and spent thousands on it back in 2001 when there wasnt any other good motocross machines besides the 250R. CURTIS SPARKS national 416 kit, 39mm FCR,ROLL DESIGN suspension, and CUSTOM AXIS shocks etc. After all of the money i spent,I gave up trying to make it compete with the new sport quads (yfz,trx450,lt-r).I broke down and purchased a suzuki LT-R and couldnt believe it. Almost instantly i was better,faster, and smoother than i ever was on the 400. Now with just a few upgrades (pipe,filter,c-bomb,ergos,fox shocks) i feel like i can be buried with this bike!!! But to help you with your respnse issue i can tell you what i did to correct my problem. first, remove the plastic side cover on the carb,observe the linkages. check to see if there is any freeplay between the slide/accelerator pump you may need to bend slightly to take up the slack.What you are doing is basically advancing the pump timing on initial throttle opening. Its a free and simple fix i have done to many 400EX and had good results. after that, just play with the fuel screw a little, go in 1/4 turn,out1/4 turn fine tuning.           

That place is no joke, better bring some b@lls! Beeline rd N of indiantown, left on Silverfox down approx. 2 miles right turn on Farm rd. After large canker wash on left you will turn right through small wash. Follow dirt rd to track. HAVE FUN!!!