400ex carb adjustment for high altitude!

[sledhed]

I have a 2003 honda 400ex that does awesome in nebraska where i am from but i recently took it to snowy range wyoming and the altitude killed the power. Im going back up there the 3rd weekend in Sept. and i was wondering if i could adjust the pilot screw maybe instead of changing jets??? Any advice would be awesome!

[gojk]

I would def rejet, but you will still feel a power loss in the higher elevation from the thinner air.

[GPracer2500]

You can adjust the pilot screw and maybe/probably you should. But that really only affects the pilot circuit. In other words, adjusting the pilot screw won't do much of anything past about 1/4 throttle.

Find out what the elevation difference is. If you know that and you know your current jetting you can make a real good guess about what to change it to. Every 5000 ft increase in elevation should need about 3 main jet sizes. That estimate includes a small correction for the typically cooler temperatures found at higher elevations.

Like gojk said, in the thin air you'll still loose power even after you're jetted right. But it won't be as dramatic as when your loosing power because of thin air AND rich jetting.

One thing you can do is take off your airbox lid (if your using it) when at high altitude. Letting the engine get more air does the same thing as leaning out the jetting. It's fast/easy to do and it will help reclaim a bit of performance lost because of the thin air.