i have a stock 400 ex and my 2000 ds (has a tm45 carb, pipe,carb mount k/n) do i just take the mains down 2 sizes on both? i think both idaho and utah dunes are 5000'.

 

I've gone from 500' to over 7000' without a problem and no jet changes. Unless you're into max hp, I would try it first. The last thing you want is go too lean and BAM, you'll be posting another thread.

 

the bigbore CV carb on the DS does not seem to be as hindered by altitude changes as some others. some small bore CV's like the duals on raptors and V's are greatly effected and do not run well at all. and non CV carbs both flat and round slides have plenty to gain from rejetting for altitude.

going down 2 sizes on both should keep you close to right, one size (2.5) increment per 1500' is a close approximation most go by, barring any other changes pertaining to temp and humidity. I drop my TM45 2 to 3 sizes from here at home in Indiana to Utah/Idaho, then another 3 sizes going up to 10K at other places.

be sure not to mistake the HP loss due to altitude for a tuning problem, it is quite considerable. Believe it or not, your stock 400ex may not even be able to climb the face of sand mountain, and will probably be down to 1st gear by the time you reach the top of the valley side. I can just about garantee it won't make it up Choke Cherry, unless you are a very very light rider, as I have watched so many fail. But being just 2 jet sizes off is enough to make or break it.

 

I just returned (45 mins ago...lol) from little sahara UT and St. Anthonies. I have the stock carb and went to a 165 mikuni main (from a 175 mikuni) and went to a 40 pilot (from a 45) I actually richened the needle a clip (had a slight lean spot) and it ran quite well at both st.A's and LS. My yfz buddies pulled their lids off and ran pretty good. One yfz buddy just put an HMF on, and ran it out there and it seemed perfect without touching the carb.