I recently purchased a 1999 warrior that was all stock. Now it has a K&N filter and a E-Brothers series pipe on it.The big question is do I automatically re-jet it or will it perform in a certain way that tells me it should be re-jetted ?

 

To quote Mike Penland, "you need to re-jet only when you need to". That said, you need to pay attention to the performance of the quad in all parts of the band; idle to 1/4 throttle, 1'4 to 3/4, and 3/4 to full throttle. If you detect any flat spots or cutting out in any of these areas, run it for 1/2 mile at that rpm, cut your engine right there and pull your plug to do a plug read. You're looking for a tan or brown color on the electrode & down the insulator. A light gray or white color indicates a lean fuel/air mix, while a dark or black color shows a rich mix.
I don't recall the elev. there in Ft. Worth, but I'm guessing that you'll find you are a little lean on the main jet with the improvements in airflow you've made. If so you need to pull the main jet to see for sure what size and type it is. The stock size is a 145, however Yamaha uses two different types of main jets in the Mikuni; one is a hex head and the other has a larger round head with screwdriver slot. Again, if you need to go fatter on the main, the next two sizes are 147 & 150. I'd get them both just in case you need them. If you do anything further to the breathing you will for sure.There is a 147.5 also but I would just skip that. It tried that & found little or no improvement over the 147. Here at 3200' I'm running a 155 on my Warrior

 

a bike will run really good when it is lean.
pull your plug out and check i'll beat that you
find that it is lean!!!!!!

 

426ex,
What you say is true..it will run really well. And really hot, until it burns a hole in the top of the piston, scores the cylinder walls, and does really expensive damage to the bottom end.(:

 

Hey ssimons, let me know how it goes. I am planning on doing the same mods to my Warrior. I am also curious about the jetting, and the performance increase with those mods.
I am at sea level, so my guess would be a 150 main?? Does that sound right??