Its running ratty; High altitude sickness?

I bought a 2002 WR426 in Ohio and it is now in Grand Junction, Colorado at just @ under 5000' elevation. It is running really ratty. I changed the main jet to the next lower size, put a new spark plug in, cleaned the air filter. When I took it out to ride it ran GREAT for the first 20 min. or so, but it was back firing when I released the gas. I stop the engine and after that it ran doggy the rest of the day. It seemed like the spark plug was beginning to foul and it was running almost at half the performance, but less frequent backfiring. Anybody know the tricks to higher elevation performance? Do the WR450's have these same problems? Thank-you so much for your replies!

You need to rejet the carb completly---i.e. pilot jet,main jet and probably the air jets that correspond to the correct jet. May also need to adjust the pilot screw (this is the fine adjustment of the pilot jet).

Re-jeting will definately fix your problem. Go to the jetting form of this website you will learn a lot!

Im at about 300ft. above sea level so my jetting will need to be richer that yours---hope I haven't confused you.

Here's what I ran with an open pipe and an FMF Q. This was for 6,000-10,000. I then just changed the main to a 152 for higher altitudes and ran a 160 main for Moab. This was on a 2001 WR426.

Carb Settings (Per LarryCo and Taffy)

- 155mj

- 38pj

- EKP#3

- 160MAJ

- 65PAJ (Adj), 2 turns out

BK Mod with 1/3 second squirt is a must as well.

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