High Altitude Re-jetting Required???

Does a YZ426 require re-jetting when riding in high altitudes such as 10,000 feet plus. I was thinking of a summer trip to the Telluride Colorado area. Years ago I was there on a 2 stroke and had to re-jet.



What year is your bike?

As was stated it will run, but you will not like it, and may even suffer a fouled plug.

I ride at 4k normally, and have gone above 8k a few times on my ’01 YZ. If you like, I’d offer the following tips:

You can keep the stock needle, but a size or two smaller on the straight diameter will help, unless you have an ’00, which is ALREADY two sizes too lean here. So if you want to really dial it order a stock ’00 needle.

At any rate, you will want to go to clip 3 at a minimum (maybe # 2). And a 40 pilot at a minimum (a 38 may work better). You can swap main jets if you want but I never noticed any difference, I haven’t done any high speed riding at high elevations.

Hope this helps.

[ January 07, 2002: Message edited by: Hick ]

Sorry, double post. :)

Get that "EDIT" button away from the "QUOTE" button, would ya Mr. Admin? :D:D

[ January 07, 2002: Message edited by: Hick ]


That's good to know. I stopped on my way back from Grand Junction to Albuqerque last may to ride up to Engineer pass in SW CO. My bike runs great at home 5k - 6k feet, but was really dogging all the way up there. Plug fouled when I got home. The road was still snowed in above 12k ft, but I decided I need to rejet next time. I'll probably ride up there again this Memorial Day WE when I head back to Grand Junction. Sounds like I can get away with air screw and pilot jet change for a day's riding???

Originally posted by John H:

Sounds like I can get away with air screw and pilot jet change for a day's riding???


To me the two biggest things are clip position and pilot jet, but I’d rather be off by one on the pilot size than one clip on the needle. One clip position in the right or wrong direction to me is much more noticeable that a size larger or smaller on the pilot.

If I was only allowed one jetting change with a (severe) change in altitude I would choose clip position (based on stock jetting). This is also convenient logistically because it doesn’t require any different parts. My second choice though would be pilot jet.

I’ve read the countless jetting tips in MXA and Dirt Bike and the like where things are supposedly improved with a slight change in the idle mix. To put it bluntly I call BS on such reports. One caveat is I don’t ride MX, but to me improper jetting on the fuel screw is manifested more by a hard to start (or easy to stall) bike more so than one that is not running properly, or not delivering throttle response to your liking. In my small brain throttle response or behavior is all about the needle.

Hope this helps.

Hey Hick,

That does help. I know I'm gonna be riding up above 9k this summer. I rode with a guy from Montrose on an XR400 and hope to look him up again. After my experience in May, I know I need to tune the carb a bit to have a good time. I recently had to pull the carb and found out how easy the jets are to get to. I guess I can just turn the carb a bit and pull the top off to change the needle clip too????? I did put a Kouba on my air screw and that is very convenient.

I see you're down south of me. If I get down that way maybe we can hook up and tear up the desert a bit. You off on weekends?

It'll run, just not at 100%. My bike is jetted for around 7K feet.

Last summer I was in Telluride and going over Imogene (13K)

was rough.

Thanks guys for the replies, this has been a great help! When you say needle clip position #2 is that from the top position down? I actually had a 1998 YZ 400 and loved it. I had knee surgery last year so I sold it. I am now considering a new leftover 2001 YZ426 that a local dealer has that he will discount fairly well.

Thanks again!


John H,

Yes, you should be able to change clip position with the carb on the bike. Really the main benefit here is you don’t have to remove the throttle cables, probably the hardest thing about R&Ring the carb on these bikes.

Also, at 4k down here in El Paso (riding area actually 4.5k and up), same basic climate that you are dealing with, I run at clip # 3. I haven’t ridden since the weather cooled down because I’m temporarily out of commission, but it may be that back to # 4 will be better…

Heck, I’ll just post all my jetting:

EJP # 3 (that is the stock needle although EJQ/EKQ may be an improvement for you since you probably routinely ride at least 1,500 ft. higher than I do)

40 pilot

160 main

about 85 on my Pilot Air Screw, an adjustable aftermarket replacement for the Pilot Air Jet, YZs have a size 100 PAJ. The smaller size kills the surging and popping at low throttle and idle.

1.5 on the fuel screw

I took the Main Air Jet out completely, I believe this was an improvement.

As you may be able to tell I’ve put a lot of time into this, over two years of seat and wrench (and Thumpertalk) time on my ’00 and ’01. That doesn’t mean I ain’t wrong, just that it is less likely. :)


Glad I could help. Yes, you count clip position from the top down, and the closer the clip is to the top the lower the needle sits relative to the slide so you are leaner across a range of throttle positions (basically ¼ to over ¾ throttle, give or take).

Yo Jay;

I live in Boulder CO, and ride my WR426 anywhere from 5K to 14K. I originally bought an '01 YZ426, and had all sorts of problems with it fouling plugs, but I think there was a problem with the bike as a whole and not just the jetting. At any rate, I personally can ride the track here on the flats at about 5K running a 162 main, and when I head up into the high country, all I do is drop down to a 158 main. I know you can get more technical with your jetting by droping the needle or installing a leaner pilot jet, but I havn't ever needed to. Just a thought, everyone one this thread is right on for sure.

Dodger :)

Thanks for the information everyone! I appreciate it and Dodger you have trail riding paradise in your backyard. I have such great memories riding in the mountains, the scenery is unbelievable!!


I'm usually spending my time on the WR side, but thought I'd chime in with my 2 cents worth...

Just rode out there last year. Ouray is fantastic! All sorts of awesome views. My favorite ride was over Imogene pass from Ouray to Telluride and back. This has now become a yearly destination for me...that and Crested Butte.

When riding there last year, I had no idea on how to jet the bike...and of course, had the same plug fouling problems. This year, I'll be much better prepared. Here in Denver, I run a 42PJ/75PAJ/EKQ#3/162MJ/200MAJ and love it. Runs great out at Rampart Range up to 10500ft.

Out there, however, I'd switch to the following:

40PJ/75PAJ/ELQ#3/158MJ/200MAJ. This is basically going down 1/2 clip position from the EKQ#3...a slightly leaner main...and going down 1 pilot jet size. I think you'll find this setup running great from 9K - 13K...

Dont know if you are aware of James Dean's jetting spreadsheet, but it's an excellent tool to use when doing just such a thing...jetting your bike for varying altitudes and temps. You basically plug in your current jetting, altitude, and temp and you can visually see the affects of changes based on varying altitudes and temps across all throttle positions. I highly recommend it. What it wont do, however, is help you dial-in your current jetting...you need a dyno for that. But once you've got that close, this tool really helps. Do a search on the WR side...I think he's asking $19.95 to help mitigate the time and expense of creating it. The best $20 I have spent...


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