I recently purchased a new WR426 and have been doing some expirementing with different mods.I installed a Baja design exh. insert with thier jetting recomendation and rode for a little while and noticed alot more snap.Then I decided to go to YZ timing and stepped up the jetting to compensate.175 mj,48pj,stock needle and position.Friend has a WR400 and he said the air box lid removal is hype.When I removed mine it didn't make an ounce of improvment.Lets just look at the phisics of the air box insert as far as actual opening size compared to the throat of the carb.The insert is twice as big. Maybe alot of you guys out there don't care about the added noise but I do.Is there anyone out there who has good hard proof other than Yamaha not installing the lid on the YZs , that there is this great performance gain? Also is there a dependable jetting referance for a WR with exh,YZ time,air box,3500 altitude and up ?

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