'01 YZ250F running too rich or lean? Listen to video

My wife's '01 YZ250F has started running like crap, mainly at higher elevations. I know engines in and out but am fairly ignorant when it comes to carbs. Everything is bone stock, the bike has only run 4 or 5 times a year since new, carb is always drained after every ride. It seems to run fine under most conditions, but once in a while (mainly up high) it will do this. She barely made it up this climb. We live at 2600' and this video was 6400'. It has done this before as low as 3300'. It is a very intermittent issue. Any ideas and possible fixes? It can be heard best around the 0:18 mark.

 

A quick n dirty test is, when it is running bad pull hot start while riding. Then try same pulling choke.

Either way it will tell you rich or lean.

Jetting does not change on its' own over time. If the bike used to run right and no longer does (which is what you imply), it is a maint. issue

Dirty air filter

Bad spark plug

Debris in carb

Old fuel

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