I RACED A 2000 400 EX WITH REV BOX K &N POWER UP KITWITH STOCK SILENCER BUT BAFFLE REMOVED WITH MY STOCK 2001 400 EX ON TAR WE SWTCHED AND EVERY TIME HIS WON BY ONE QUAD LENGTH WOULD PUTTING A SLIP ON PIPE DO IT?

 

Have you changed the jet's in yours? He has a jet kit, a lip-on will help alot from stock. But, first jet the 400. The main should be arond a 152 or 155 stock, pilot 40 or 42. This alone will make a big difference. The pilot will help on the starting.

 

a slip on with k&n and rejetted helps a bit, thats what i did to mine it doesn't really make it that much faster, but i think a rev box thing helps alot.