Sep 6, 2000
High altitude jetting

Thanks James, you explained it much better than I ever could. I raced the Over The Hill Gang Mammoth Mountain MX national last week and did a lot of
high altitude (9,500ft.)jetting R&D while I was up there. There were Three 520's and three 400's that we jetted for altitude. We tried 92 octane fuel at first, then went to 87 octane and had much better results with the lower octane gas.
(since it burns faster, the bike runs a little leaner, which in turn means less drastic jetting changes). Then we proceeded with the stock needle in the #2
position, with a 170 main jet. This was the favorite of everybody except me.
I ran best with the OBDVR in the #4 pos. with a 168 main. Clip pos. #3 was a tiny bit lean, and even though the bike reved quicker, it seemed to breath a little hard, and since I was racing, I didn't want to take a chance and run too lean. On the big uphill, I was able to pull on everybody except the cr-500's and KTM 520's. By the way, with the help of my trusty and well tuned 400sx, I was able to pull two holeshots, and three first place moto's and one second place moto for the overall win in my class. :cool:
I didn't spend any time with the e-series needles, as I felt like the steeper taper would have made the bike richer, and I would have been going in the wrong direction as far as jetting was concerned. 1 and 1/4 turn on the fuel screw was best for just about everybody up there. I tried a 45 pilot instead of the stock 48 and that was a joke, so I went back to the 48 . Erik
Top Bottom