'02. Yz250 running rich ? Help please

Float height set correctly?
Float needle and seat sealing?
Top end fresh?
Compression still good?
Crank seals good?

Paw Paw

It's a perfectly fine question because jetting can get confusing.
First, what Paw Paw mentions above is super important (he knows his shtuff!!) because
trying to chase jetting on bike with worn parts or not at a proper base line is a waste of time.

But after that, you can dig in. This is a pretty good video that explains the basics:
https://www.youtube.com/watch?v=Uid8L4yMvfc

A more techy look:
https://www.millennium-tech.net/pdf/Jetting_Basics_for_2_Strokes.pdf

Rick, move the cir clip on the needle up to the next slot, this will let less fuel (more air) into the cylinder. If you reach the top slot on the needle and it’s still dripping black gunk it’s time to move to a smaller main jet.

That's not how needle clip position and main jet size relate at all.

Nothing personal, but that second sentence is not going to help anyone jet a bike properly.
There are diagrams in every manual that show how jets relate to throttle position.

Generally speaking, the stock YZ250 jetting is very good. 50P, 178M, N3EJ needle in clip2. This assumes you are running 32:1 ratio. Ratios with less oil will richen the jetting and cool combustion temps causing excess spooge.

It also assumes everything is tight, clean and correct in the carb as PawPaw said above. A sticky/leaky float needle can throw this off. A sticky/leaky/degraded power jet solenoid tip can throw this off.

This also assumes good, fast MX riding where the combustion and pipe temps are kept up and the engine is kept cleared out. Slow, tight, trail riding or slow/novice MX riding will have the bike running too rich.

On a YZ250 this old, a leaking right-side crank seal is a real possibility. This will allow transmission oil into the engine. However, although more oil is coming in, this usually presents as a lean condition as more air is also getting sucked in. The bike will usually feel extra zingy. Almost too snappy.

All that being said, yes, it can be as simple as replacing the pipe o-rings. My YZ250 is jetted great and runs very clean and crisp. But it will leak black gunk at the pipe junction if I let the o-rings get old.