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I recently purchased a 2008 and live in the Sierra Nevada foothills of California. We live at 1500 ft. elevation. I recently re-jetted the bike but kept the stock exhaust. Commonly, the main jet is changed to a 122.5 with a 38 pilot jet. I decided to try a 120 main/38 pilot. The CRF seems to pull very well and runs very clean. I decided against the larger main jet because I intend riding at elevations over 4,000 feet and wanted the bike to run relatively clean. I will likely pull the air box lid for riding at higher elevations.

Welcome to the site RR_rpf.

If you find your bike works well with the jetting you have chosen then I would stay with that until you have had a chance to try it at 4000 feet.

Not sure if you will get away with just removing the top of the airbox at the higher altitude, you may need to make some adjustments to the idle mixture screw as well. If you are changing altitude on a more permanent basis then a small change to the jetting maybe required.

With the 38 idle jet how many turns out is your idle mixture screw?
I ask because it seems a little small even for an uncorked bike. It might be standard but the jetting tends to be lean from the factory.

Let us all know how you go.

My crf230l carb is now using 38 and 125 jets with stock exhaust. Fuel screw at 3-1/2 out. It runs perfect here in the tropics. I installed larger (but not so large) jets because I have a spare T4 exhaust (full system). Bike seems running ok with either exhaust. But haven't tried riding in very high altitude, may 1500 ft at highest.

Hi Benjamin,
Welcome to the site.
If your fuel screw is 3 1/2 turns out I would up the idle jet to a 40.
If your more than 2 1/2 turns out the screw can fall out.

Hi Ray,

Thanks for the welcome.
Sorry can't really remember what the slow jet was but am sure the main was 125. Yeah, at these jet settings I had the fuel screw 3-1/2 out and it's where the rpm peaks at idle, and i don't think it will fall off as there are still many threads engaged. This bike of mine is actually designated as XR230L, being a Japanese or asia model, but its completely identical to the crf230l model marketed outside Asia.
Grey bike imports from temperate countries, when these are used here in PH(tropical weather) need to have the screw at least 3-1/2 out or else the bike often stalls. It has something to do with the difference in air density between temperate and tropical weather..