How much of a difference does...

[inanecathode]

Rejetting a bike for altitude make?
Im fairly certain my xt250 was never jetted for, at least colorado, but more certainly my altitude (i ride it 9000 feet and up when i trail ride). It hesitates, stumbles, and won't freakin start (when i get up to around 10,000 feet) when i ride it on the trail. On the street it's got good acceleration up to about 35mph, then it just won't go.
I'm pretty sure rejetting it will solve the hesitation, stumbles, and poor starting, but will it give me more power when the throttle is pegged?
Also, do you guys think i should do just the main jet, or all the jets?

[kwells]

I think increasing the pilot air jets will help

[inanecathode]

Increasing or decreasing? You reduce the opening to reduce the fuel to cope with less air.... right?

[Brian Moffet]

He's opening the air jet to deal with less air.

[kwells]

H20 and I are running 135's for pilot air jets but were discussing the effect that ~137s might have also.

[Coil Coyle]

Increasing or decreasing? You reduce the opening to reduce the fuel to cope with less air.... right?

Cathode,
            Start on page 4 to get a foundation to build your understanding.
http://www.mikuni.com/pdf/vmmanual.pdf
Learn Pilot and main theory, they are all the same idea

$0.02
;)
Coil

[inanecathode]

Oops, lol. Wasn't reading. Dunno what i read, but i didnt read right. I always thought you drop fuel jet sizes to deal with less air?

[h2olawyer]

Either way may work.  At altitude, there is less air density, so you can either restrict fuel or increase air.  I dropped the main jets to 120 F & 125 rear once as a test.  Couldn't make it run right at all.  The stock (upgraded) jetting works really well from 1,000 to about 9,000 ft.  That's why kwells & I have discussed going to a 137.5 pilot air jet.  Inexpensive test & easy to reverse.

H₂O