400ex jetting at altitude

Honda

I am taking a trip to taylor park colorado and taking my 2004 400ex since my bike just lost a lower rod bearing. It is bone stock and I am wondering what jetting changes I will need to make since the altitude will be above 9000ft. Anyone know if I can just get away with changing the main jet and playing with the fuel screw?

thanks

It really just depends on what altitude your starting from. If you make no changes it'll run way too rich. If the altitude change isn't too dramatic then you may not even need to adjust the air/fuel screw, if it is then an air/fuel adjustment will probably get you bye. And you'll almost certainly need to move up the clip on the needle. Some main jets, a couple of pilot jets, a couple of screwdrivers and a pair of needle nose pliers plus 10 minutes is all you'd need to dial it in, so just take them with you and adjust it when you get there.

I am starting in Texas so it will most certainly be a change in altitude. I was thinking I might be able to get by with just a smaller main jet and needle clip change 1 or 2 positions higher. I have never torn apart the carb, does anyone know what the stock main jet is?

an air/fuel adjustment will probably get you bye. And you'll almost certainly need to move up the clip on the needle. Some main jets, a couple of pilot jets, a couple of screwdrivers and a pair of needle nose pliers plus 10 minutes is all you'd need to dial it in, so just take them with you and adjust it when you get there.

The mixture screw is ONLY for the bottom RPMs. Suggesting that that may be all he needs is just wrong. You'll also need a main jet change before a needle movement because the main adjusts the mid too. The needle just "fine tunes" the mid range and probably won't have to be touched.

Okay, just have a 17mm wrench for the access hole on the bottom of the carb, a small flat head screw driver for the mixture screw, 6mm socket for the main jet and a couple smaller main jets. You shouldn't need a pilot jet because the stock pilot is a tad lean already and you can turn down the flow enough with the mixture screw.

Don't waste your time! Put a couple Thunder TPI valves in the airbox lid. Open them as you ascend, and close them back down again as you descend. Above 9,000', just pull the lid off altogether. This is indeed a rejet, it is just that rather than cutting down the amount of fuel in the mixture, you are adding more air instead!

My typical mountain rides go up to 12,000', and I am jetted for sea level (I even sometimes ride below sea level). The system works great!!!

Recon's way can work. but if you want to just change the main jet, just drop it 3 sizes from what you are running right now. Just don't run lower than Taylor park. And when you are up there , you may need to adjust your idle mixture screw and bump your idle speed screw.

Don't waste your time! Put a couple Thunder TPI valves in the airbox lid. Open them as you ascend, and close them back down again as you descend. Above 9,000', just pull the lid off altogether. This is indeed a rejet, it is just that rather than cutting down the amount of fuel in the mixture, you are adding more air instead!

My typical mountain rides go up to 12,000', and I am jetted for sea level (I even sometimes ride below sea level). The system works great!!!

Not a bad idea at all.:bonk:

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