Originally Posted by Spud Rider
One doesn't need to adjust the pilot mixture screw for different altitudes. If your bike has been running well, your pilot mixture screw (PMS) is probably set correctly.
I am using a size 55 pilot jet, and a size 155 main jet in my XR650L's carburetor. My bike runs well from 4,600 to 9,200 feet above sea level. Another XR650L owner has reported his bike runs well from sea level to 14,000 feet above sea level using the same carburetor jets. However, a lot depends on whether you have performed Dave's Mods, removed the air box snorkel, et cetera. Neverthless, the constant velocity (CV) carburetor on the XR650L does a pretty good job compensating for altitude.
What jets are currently installed in your carburetor? You might want to go a size lower, at least on the main jet, when you begin riding at 13,000 feet above sea level. You can change the carburetor jets without removing the carburetor.
I suggest you see how well the bike runs at high altitudes before you decide to change carburetor jets. If you need any help, don't hesitate to ask. We can tell you how to change the jets easily.
We de smogged, de snorkled, dave's modded, and put on the UNI. I can't remember what jets we put in; I want to say it was 58/155? Lee, maybe you could chime in?
I have the factory jets which are a lot lower if I remember correctly, but I'll have to look at them to see how big they were. I don't remember off the top of my head.
I do know how to re-jet; however, I didn't think about doing it with the carb still on. That's makes sense. Thanks for the idea.
I had heard that you could adjust the mixture screw to comensate a little bit for altitude, maybe you can elaborate on that? Is that true?
The bike runs weak at high altitude; i've had it up to 10,000 feet a few times now and you definetly can feel the sluggishness. It also doesn't idle well at high altitude. If you don't keep it reved a little while you are idling, it tends to die pretty quickly.
Like crashmaster said, I'm about to start getting really high. Not sure what kind of adjustments I want to make yet (if any), but I'm looking for ideas.