I used to hang here in 1996-1997 when I had a Sport 400L(Piped, clutchkit ~82 MPH) anyway it was stolen. I bought a 1998 XR400 then a 1999 YZ400. After a while I missed my 4-wheeler. I sold the YZ and bought a Y2K 400EX. Now I am back. I have had my EX for about 4 months and I was ready to mod it...

Yesterday, after much reading, I went to the local cycle shop and purchased a FMF Powercore IV pipe and a K&N Filter,outerwear, and Jet Kit. The first thing I found was that my throttle was not opening the carb all the way. I fixed this with the adjusting ring. Next, I installed the pipe and filter/outerwear. I read the Jet Kit instructions and I said for: Pipe, Filter, airboxlid on use a 146 Main for my altitude. It comes with a 148. So I left the jetting alone and put the 2 front clips under the lid.

This thing has much much more bottom and mid hit. I could not test the topend last night so I will update with that later. I imagine that with the airbox in it's current form top end will be just a little better. Keeping the front end down is truly a challenge. Thanks to all the posters who's info helped me make my decisions... The Jet Kit is going to be returned. I am gonna purchase 160-170 main jets and a 40 or 42 Pilot. Possibly a needle from the Honda Hop Up kit. Then I can remove or vent my airbox for the sand.

Thanks to everyone. I look forward to contributing where I can.

Kendall

 

I've ridden a Power Core IV-equipped 400EX and I really like the power. I'd buy one for mine, but man, it's way too loud! I'm holding out for the new FMF system. They claim it has same performance and is quieter than stock. I'll believe it when I see (hear?) it.

The Power Core IV adds good power, no doubt.

 

IN RESPONCE TO YOUR DYNO JET KIT YOU BOUGHT.
THE STOCK JETTING IS #148. AND THE DYNOS IS #146
THAT IS THE CORRECT ONE FOR ALLTITUDES BETWEEN
SEA LEVEL AND 1000 FEET THE DIRECTIONS TELL YOU TO USE THE ADAPTER IN THE KIT THEN PUT THE
#146 IN , USE THE NEEDLE THEY SENT PUT THE CLIP IN THE 2 OR 3 NOTCH, THEN TURN THE AIR
MIXTURE SCREW OUT 3 HALF TURNS OUT. THE DYNO
KIT IS A TOTALLY DIFFERNT SET UP.