Jetting For High Altitude

Just wanted some input from you guys. I have a trip planned in UTAH in about a week and was wondering if the stock jetting is going to give me any trouble. I have had my bike for a little over a month and ride 1-2 times a week absoulutly no problems at all. The ALT here is about 500-1000feet where i ride but in Utah will be between 4000-5000. ? how would you set up for that ALT from stock jetting. Like I said with the exception of some minor suspension adjustments the bike is the same as it rolled of the floor.


I've taken a 98 & 99 YZF400 to Colorado & ridden at elevation of 8,000 to 11,000 ft with no problems using stock jetting. I would leave it as-is.


Thanks just didn't want a poor running 426 to ruin the fun.

I would take a strong look at re-jetting for high altitude, while your bike will run, it may not run as well as it should. If you leave the jetting stock, carry extra plugs and get fast at changning them, especially if you are going to be riding tight woods.

just my .02


P.S. do a search on high altitude jetting and you can watch the pain I went through.

Stock jetting will be passable, but it will run much better with leaner settings, particularly at this time of year.

The biggest difference will be clip position and pilot jet. One leaner on each should be very close at that altitude. The clip pos. you will notice much more, it affects a wide range of throttle. The pilot you will mostly notice at lower rpms and throttle response off of idle, plus it may prove more difficult to start w/ the stock 42, particularly after a stall.

I also ride at 4k to 5k or so and in a similar climate most of the time, so here is my jetting on my ’01 w/ stock motor:

160 main

40 pilot

EKQ needle, # 3 (stock is EJP, so I’m 1.5 clips leaner than stock)

80 pilot air jet

1.5 turns on idle mix

BTW stock jetting on a YZ 400 is much different than any model 426, not to mention the extra 26 ccs.

Hope this helps.

A good rule of thumb is to go 10% leaner, between sea level and 6K' anyway..


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