Jump to content
Sign in to follow this  

high altitude jetting for xr400 7,200 ft +

Recommended Posts

hello folks. i just bought another xr400, a 2004, being out of it awhile. the bike originally came from california, but was set up for colorado. the bike runs great, starts easy, but the usual crack the throttle, it falls flat on its face, but if i ease throttle back out, it comes back alive, and thats the only time it does it. i called a local dealer, and read some back issues of dirt bike. they said this for the jetting. 138 main--stock 52 pilot, needle-2nd clip from top, air screw-2 turns out. any other suggestions before i go with a pumper carb? i just want it to work easier of the bottom throttle. the bike is 100% stock otherwise. i will be riding at my elevation at my house is 7200 ft, and up at rampart range to 9,000 ft. thanks for the help!!!

Share this post


Link to post
Share on other sites

The 52 pilot jet is too rich for a completely stock bike for those elevations. I would try a 48 and readjust the fuel screw for best throttle response and idle.

I'd also drop the needle clip back to the middle position(3rd from top).

How does it run above half throttle?

Share this post


Link to post
Share on other sites

thanks for the response. it runs great all around, other than dead spot when i hit the throttle fast from idle. if i am moving slowand crack it open with alittle higher rpm, its ok, just from idle on a fast roll of the throttle.

Share this post


Link to post
Share on other sites

i think i want to keep it stock, but would it help alot by replacing the muffler end cap with a more open unit, and the top of the airbox removed? just dont want to make it even more difficult to jet. thanks.

Share this post


Link to post
Share on other sites

If you do "at least" the Gordon mods, it will run better at your elevations and will be easy to jet for, mostly because there are so many people with experience for that configuration.

You don't need another muffler or end cap. The stock unit is a good unit, just needs an easy mod.

Yank the snorkle in the top of the airbox and replace the stock air filter and cage with a UNI.

For that configuration at your elevations a 150 main and 50 pilot with the needle in the 2nd from top would run well.

Be aware, whacking the throttle open quickly from an idle won't get you 2 stroke-like response. It's nature of the beast with the XR 4 stroke and stock carb, but most hesitation can be tuned out with careful fuel screw tuning.

Don't alter the exhaust unless you also install an aftermarket air filter like the UNI. If you mod one you have to mod the other.

Otherwise, keep it stock and just lean out the pilot jet with a 48 and retune the fuel screw for best idle and throttle response.

i think i want to keep it stock, but would it help alot by replacing the muffler end cap with a more open unit, and the top of the airbox removed? just dont want to make it even more difficult to jet. thanks.

Share this post


Link to post
Share on other sites

thanks for the help. i was getting most of the past info from 2000 dirt bike xr400 test when they did it from colorado. i think stock sea level was 142 main and 52 pilot, needle in middle. local honda set it up with 138 main-52 pilot, needle 2nd from top-riding at 7,000+ altitude. airbox lid, filter, and drilling the baffle would really need to take it to a 150 main? i know thats not much compared to everyone else, but just want to get it pretty close. also, the bike came from california--is the needle different? should i buy another needle? it an 04 model. thanks triailryder42 for your help. am just trying not to get mad at the bike to get it perfect, or where it should be. your opinion at stock though would be a 48 main and needle back to middle? i will probably do the gordon mods though, and worse case--buy a pumper carb from xr's only. thanks again!!!pappy.

Share this post


Link to post
Share on other sites
i will probably do the gordon mods though, and worse case--buy a pumper carb from xr's only.

I don't own a 400, but I can tell you that the G mods on a 250 made my engine run like a whole different animal. $30 for a UNI & some spare change for a grinding stone (header welds) & drill bit (exhaust). New jets. I also replaced the resistor in the spark plug boot with a hacksawed stainless steel screw. Very cheap for performance gains that'll make you grin. I'd do the G mods first, then decide if you still need (or want) a pumper. Nothing wrong with keeping it stock if that's what you want, but your beast will remain caged. Just my 2 cents.

PS. I hope you post some riding photos of that beautiful state you live in! :)

Share this post


Link to post
Share on other sites

I have a 97 xr400 and I ride at rampart and others even higher. Bike is mostly stock with gordon mods. I bought a handful of jets and think I ended up with 140, 48. I will check to be sure and post tomorrow, but mine runs great. Even went to the top of mount evans last week at 14265.

Share this post


Link to post
Share on other sites

Yes, the Ca. needle is different. Sorry, didn't think of that. The 49 state needle is an A16A needle. A good upgrade.

Seems like the local shop set it up for riding on the main, but in Colorado you need good low end and off idle response for the single track and slow technical stuff. That 52 pilot on a stock bike at the elevations you're talking about is Blubber City.

How many turns out from seated is the fuel screw currently? That will help tell you if your pilot jet is rich or lean. Ideal fuel screw position is 2 turns out when the proper pilot jet is installed. On your stock bike, with a 52, at your elevation, if it's running half way decent at all I'd say the fuel screw is set less than 2 turns out, I would think closer to 1 turn out, to lean out the too rich pilot circuit.

I'd put the 48 pilot in it. Leave the Ca. needle position where it's at for now. You don't want to make too many adjustments at once in different jet/throttle ranges. It's the off idle range you're having fits with for now, so work on that.

As for the 150/50 jetting after the Gordon mods, they let the engine breathe sooooo much better, so the jetting has to richen up for the increased airflow the engine is getting. If you notice, I said 50 pilot "after" the Gordo mods, so that tells you that 52 you have in it now is way rich.

Sure, the XR will "run" with overly rich jetting and take you just about anywhere you want to go. I guess it just depends on how picky you are about the performace you're getting out of it, the types of riding you do and how aggressive you are. For me, if I feel it's running rich and I'm getting any blubbering/hesitation at all when I'm trying to ride trail, I'm changing jetting and retuning. I don't like lack-luster response.

These suggestions are just my experience from having and tuning my '00 XR4 for the last 9 years. Hope you get yours figured out.

Share this post


Link to post
Share on other sites

I just checked on my jetting and I have 145/52. Once I got it jetted i haven't touched it and it runs well in summer and winter at all elevations. That is not to say that rejetting every time the temp or elevation changes wouldn't give better results, I'm just not into that. Also these were the smallest of the jets that I bought, so maybe try those and a few smaller, bigger is worse. Oh, and my bike is a 98. I was tired when I wrote the first post.

And I do have the a16a needle in the middle clip.

Share this post


Link to post
Share on other sites

0000 ft > 160 / 60

1600 ft > 158 / 58

3200 ft > 155 / 55

5000 ft > 152 / 52

6500 ft > 150 / 50

9800 ft > 148 / 48

with gordon mods & iridium plug i run 150/50 from moab up to 11k ft.

maybe 148/48 in stock form for you, but the mods are worth it!

adding a fuel screw is THE key for coloardo riding!

http://koubalink.com/fuelscrew.html

it fine tunes the pilot jet (the 50 number)

i turn it in when up high to restrict fuel/lean it out & turn it out when down low to richen things up.

pilot jet controls fuel at low throttle, main jet controls fuel at high throttle, needle shape & position works the middle stuff

the blubber on xrs come from incorrect/rich pilot jets. there is no perfect jetting for all riding conditions/elevation but anyone who rides colorado needs a fuel screw (and an denso IX24 iridium plug to ignite out of range fuel mixtiures, plus help starting) and the tunablity it affords.

hello folks. i just bought another xr400, a 2004, being out of it awhile. the bike originally came from california, but was set up for colorado. the bike runs great, starts easy, but the usual crack the throttle, it falls flat on its face, but if i ease throttle back out, it comes back alive, and thats the only time it does it. i called a local dealer, and read some back issues of dirt bike. they said this for the jetting. 138 main--stock 52 pilot, needle-2nd clip from top, air screw-2 turns out. any other suggestions before i go with a pumper carb? i just want it to work easier of the bottom throttle. the bike is 100% stock otherwise. i will be riding at my elevation at my house is 7200 ft, and up at rampart range to 9,000 ft. thanks for the help!!!

Share this post


Link to post
Share on other sites

thanks for all the replys!! 1st, i am trying to find the a16a needle--no one has any here? were you also leaving needle in middle or 2nd from top? thanks again!!!

Share this post


Link to post
Share on other sites

hey guys, I'm trying to help a friend jet his 400EX quad and from what I understand it has the same engine as the XR400. If that's correct we should be able to apply the info from this thread. It has an FMF powercore 4 slipon and a K&N filter, and last night we opened up the carb to find a 155 main jet, which I think is too lean for our altitude of 600 feet. We're leaving for Colorado in 2 weeks to do a lot of riding from 10-12k feet and we need to get his jetting right here before we try to jet for the trip.

Share this post


Link to post
Share on other sites

ok, i found a a16a needle today, along with a uni filter. drilled out the exhaust-per gordon, airbox lid off, and uni installed.

a16a needle --2nd from top

pilot still at--52

main at--150

2 1/2 turns out.

runs great on top., but almost just as bad from the bottom as before?

should i go to the 3rd clip, or just try the 50 pilot?

i bought a 50 and a 52 pilot jet

also, for mains i have a 148, 145, 138, 142, and the 150 thats in it. is my problem only in the needle-pilot, or air screw?

when i was riding it going from a little more than an idle, i roll it on, and i almost go over the bvars until i let off. i am not wacking it open fast.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×
×
  • Create New...