Need Help with High Elevation Jetting

Hello, I live in Delaware and I ride between sea level and about 2000 ft. at Tower City PA. This August however I'm going to Crested Butte, CO to ride for a few days with a friend from Aspen. My friend tells me that we will be between 8000 ft. and 12000 ft.. I have an 01 YZ 426 with stock jetting.

Do any of you ride at these altitudes, and if so, can you help me with what jetting to run?

Thanks a lot.



May want to post this on the WR side. There are quite a few TT'ers out there and could probably help.

You could also search the subject, use jetting and co or colorado for the search criteria.

Have fun



99 WR400f, YZ timed, MX-Tech suspension, Scotts steering damper, White Bros E-Series (12 discs), tapered header and a/f. Kouba T-handle for the fuel screw. Works Connection billet throttle tube and frame guards. Cycra Pro-Bend, triple clamp mount handguards. Thumper Racing rad guards, Renthal Jimmy Button highs, YZ Tank and IMS seat, YZ number plate, odo removed, EKP, 48PJ, 175MJ at 500-1000' Thanks James Dean!

I'm curious about this too. I'm heading up to between 3 adn 6 thousand feet this weekend and was hoping I could get away with leaving my setup as is (stock). I'm pretty new the the moto mechanic stuff and have never jetted a carb...don't want to yet either. Sounds like you are going way up there and will difinitely have to change something.


Sup' John;

I live in Boulder CO, and all my riding is above 6000ft. If your lucky, you won't need to do much for jetting changes. You may need to lean out your pilot screw a 1/4 or 1/2 turn in. May also need to drop your main jet, stock is 162, may need to go down a couple of settings. Fact is, it's hard to say. I have buddys in Breckenridge that have never changed their stock jetting and have never experieced any problems. However, in my case, I've owned my bike for a couple of months, and it seems to be much more finicky. In fact, I've run the gammot with jetting changes, even tried suggestions from shops in the area. I'm affraid I may have a more serious problem, I get my bike back from the shop today, it's now the shop responsibility to make it work. Have fun riding the mountians, it's awsome out here.




[This message has been edited by mikeolichney (edited 06-28-2001).]

I am also in Boulder and run a 40 pilot, stock needle/clip, 155 main on my 01YZ426 for our 5000' ASL track. Wintertime I switch to a 158 main, 42 pilot. I don't ride the YZ at higher elevations. For some reason, my wr250 likes a richer pilot (45) for 5000 to 8000' ASL. I have a place in CB and was just up there with it. I could not get my WR to stop backfiring with the fuel screw above 8000'. From what I can tell, the pilot will be your problem, but it is really just an annoyance. I didnt foul a plug. I recommend starting with the 42 pilot and go from there with the fuel screw. Try a 152 main and stock needle.

I have a altitude/temperatur jetting Excel spreadsheet if you want me to email it to you.

Mike, that would be great if you could e-mail me that jetting chart. I had a KX 250 that told you in the owners manual how to jet for altitude.

Thanks again.


Just spent 5 days riding in Mammoth between 8000 and 10000 ft. Two of us had '01 426's. Did not dial them in perfectly, but found a that a 158 main, 40 pilot and clip 2 or 3 from the top worked pretty good. Temp was in the 70's.

You might also consider trying VP C-12 fuel to compensate. Since this fuel tends to richen jetting anyway, going to leaner jetting with this fuel might help you retain throttle response and overall engine power. My 426 absolutely loves this fuel!

Boit, what is the octane of this gas? I was running 112 cam II in my KX 250, and it loved it. However, I've heard that the 426 doesn't like high octane gas. Which doesn't make any sense since high compression motors generally like higher octane...

Thanks for the advice,


The C-12 is rated at 108 octane but I'm not sure what method is used. The VP site won't load for me to check. The octane rating is not all that important for this application. C-12 definitely has adequate octane. What is most important is the distillation curve. With such a high-revving engine, the fuel needs to be matched correctly and C-12 accomplishes that. Again, if you are willing to jet sharp, the rewards are significant.

Boit, what jetting do you have in your bike?

What elevation do you typically ride?

Thanks for your help!!


I just raced my 00 426 in the Pikes Peak Hillclimb, elevation from 9,000 to 14,110. I can't remember if our carbs are the same but I ran stock needle, pilot, fuel screw, with a 155 main. Yes it pops on decel, always has. Still on the original plug and it RRRIIIIPPPPSSSS!!! These bikes seem to be very compliant to jetting changes. Just bring it out and ride it and don't worry about it so much.

Roostn in Golden

I'm at sea level and use a #158 main during the warm months and a #160 during the colder ones. I also installed the Fatory R&D P-38 accelerator pump plate. I couldn't ask for better carburation now.

Roostn, Pikes peak sounds pretty main jet to me. How long did it take you to get to the top?

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