I have an 02 Yamaha Warrior. I currently have brand new razr tires, K&N air filter, a cobra pipe and jet kit. I ordered a Jardine rt-4 exahust. When I put the Jardine exhaust on, then if I re jet, get a rev box, the outerwear air box cover and prefilter to go on my K&N airfilter, and put on 14 tooth front sprocket on..Do you guys think I will be able to keep up with a stock honda 400ex or z400

 

on starts but there still gonna pull away try big bore kit cam and stroked crank u should be able to kill everything oh dont get the pre filter lid drill 6 1/2 holes in top of stocl lid and plug with foam cheaper and works beter

 

If you can ride a lot better than they can, you may still be able to keep them in sight. Otherwise, you better work on your riding skills to keep up. I used to have to kill my Warrior, even scareing myself sometimes out on the trails to stay right on their heals. Even the 450s couldnt pull away on the trails when I had about the same mods you mentioned. The straight-aways were a different story. I wouldnt waste the money on the rev box. Youre not gonna gain anything from it. If I were you I would get a good cam and valve springs, port & polish the head, install larger valves, and install a 10.5:1 JE piston. Then you could beat stock 400s. What kind of riding do you do?