SOURCE: 2008 yamaha yz 125 carburator
You say it's flooding - is fuel coming out the overflows? If so, you might need to replace the needle and seat, but based on the age of the bike, I wouldn't think you would have to do that yet. Even with a bike that is raced, you can get many, many years on the needle without having to replace it - unless something damaged it. Inspect the needle and seat closely for nicks or imbedded **** in the sealing area. Also - I assume you set the float properly. This is an important step. Personally - I think the factory setting is too high - since about 2001 I have set the float 2MM lower than recommended. Otherwise you will have fuel pouring from the overflows on the faces of jumps or in the whoops.
If the engine is "flooding" - it could be down on compression which will make it run rich. Or you might just have to re-jet but I would only re-jet if the engine is fresh. You can also try running a B8ES pluf - it's a step hotte than stock and what I ran. Keep in mind that the ring in a 125 doesn't last long at all. There's only one ring. I would replace the ring before each race when I was running 125s - this happened every week during the season. 2-3 ring replacements per piston, 4-6 piston replacements per cylinder.
If you provide more info, I can give you a better answer on your exact problem - I don't know if I really know exactly what the problem is.
Scott
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