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I am at wits end with this problem. Here are the vitals

Bike- 04 CRF 450 R

Conditions- Altitude from 4300ft Salt Lake City to the mountains at 7500ft, humidity around 37%, temps around 80-100 degrees, trail riding.

Modifications- Thunder Alley full exhaust with quiet core insert.

Current Jetting- 165, 42, and the clip is dead center.

Running Issue- Whenever I ride in the mountains, give a burst of throttle, then pull the clutch in, it almost stalls or does stall. If I wait for about 5 seconds the idle comes back up. I've been told I am way too rich. I think it started around the time I put the exhaust on and I have never touched the jetting. Playing with the fuel screw does nothing to solve the issue.

Problem variances- The issue seems to go away when I come down from the mountain to an altitude of around 4400ft. In fact, I usually have to crank the idle up while I am up the mountain, and by the time I am back down the idle is really high. The quiet core insert made the problem worse when I installed it.

Thanks for your help!

Your description does make it sound too rich. The main thing that changes with lower altitude is the presence of more oxygen. The JD kit has a needle and setup especially for altitude. You could give that a try. I suggest that you call him and inquire.
Joe

We have the JD kit in Pat's 250r, and it runs flawless up to 9500'. I just drop the main one size for the KM trips only, then tweek the fuel screw at camp warmup. usually just in a little bit.

paul

Thanks for the input guys. I was trying to get away without doing the JD kit. I went to a 160 main and up one on the clip. If that doesn't work I will get the JD kit. Thanks again!

what i have found is usually a main jet is all you need to adjust at higher elevations. a 160 should help.