Rebuild the carb? It seems like a very common bit of advice to dirt bike performance problems is rebuilding the carburetor. I think it's in second place, right after "check and/or replace the sparkplug".
The reason a carb rebuild is recommended so often, is because it helps so often. Have fuel pouring out of the carburetor? Rebuild the carb. Engine bogs when you crack the throttle? Rebuild the carb. And so on.
The good news is a carb rebuild is one of those things that seems complicated but is, in fact, simple to do.
First, try and find yourself a clean area to work in or on. There are some small parts, and if you drop them, you'll want to be able to find them easily. Also, you're working with leftover bits of gas and good amounts of degreaser -- so the dining room table is probably not a good idea.
If you can, get hold of a shop manual for your specific dirt bike. That way you'll have a detailed description and diagrams for disassembling and reassembling your carb, plus any settings. If you can't get a manual, go online to someplace you can order parts and print out the exploded view of the carburetor many of these sites have. Also, I find my digital camera valuable for taking pictures of things BEFORE I take them apart. Later, if I can't remember which way something goes, I check one of the before photos.
For the most part, all carbs are pretty much the same and rebuilding the carb on any dirt bike is pretty much the same. If you're into some real vintage dirt bikes I may have a missing or extra step or two here -- but you'll probably figure it out.
So let's get started. First, shut off the fuel from the tank. You wouldn't believe how often I forget that step and pour gasoline all over the floor. When the fuel is shut off, disconnect the fuel line from the carb -- keep a rag handy, because you'll get a little residual fuel coming out of the hose.
Now take the carb off your dirt bike. Usually this involves disconnecting a rubber or plastic sleeve that runs from the air filter to the carb. Some carburetors have a small foam air filter fastened right to them -- common on pit bikes. If you can take it off before removing the carburetor... it'll just make everything easier to handle. Then, if you have the clearance, remove the top of the carb (where the throttle cable goes in) pll the slide out, and leave the slide hanging on the cable. If you're not going to remove and clean the slide and your shop is dusty, slip a plastic bag over it. Then remove the carb from your dirt bike's engine. If you're tight on room above the carburetor, remove it from the block first, THEN remove the top as you move the carb off to the side and down.
The bottom of your dirt bike's carburetor -- the bowl -- is full of fuel. Open the drain screw at the bottom and drain the fuel. By the way, if you're a smoker, you may have already discovered it's a bad habit to indulge in while doing this.
Moving right along... remove the screws that are holding the float bowl on, then remove the float (being careful not to bend anything), and the float needle.
Then remove the idle adjust screw BUT FIRST, turn it all the way in and count the number of turns it takes 'til it stops. Write this down and when you reassemble the carb, turn the screw all the way in and then back it out the same number of turns. That'll give you a good starting point for adjusting the idle later. Do this with any other adjusting mechanisms or screws your dirt bike's carburetor may have.
Remove the choke assembly. It's not necessary to remove the butterfly valve/flap, but on some bikes it pops right out. Choke assemblies are one of those things I always take pictures of before disassembling because I'm lousy at remembering how they go back together.
Finally, remove the jets. Those things that were hidden by the bowl, sticking out from the main body of the carb, with slots in them for your screwdriver.
At this point you have a couple of choices. Some people like to spray the carb with an aerosol carb cleaner (can be very messy), some like to dunk it in solvent and scrub with a small brush (gets a little messy, and takes some work), or some like to soak it overnight in commercial carb cleaner (easy, not too messy, a little more expensive). For me, the extra cost of the carb cleaner is worth the convenience and the shiny, brand new look of the clean carb.
All the loose parts need to be cleaned as well. Do them the same way you choose for the carburetor body. Don't forget the carb slide that you left hanging on the end of the throttle cable.
Don't poke or scape with any metal objects -- especially any holes in the carb body or jets.
When everything is clean, blow it all dry with compressed air if you can -- otherwise, dry it as best you can and give it a couple more hours to let things evaporate.
Reassemble everything in the reverse order you took it apart. If rubber gaskets look worn or torn, replace them.
If you have the specs, or if you were having fuel delivery problems, you can check the float level. Personally, I never worry about the float level unless I'm installing new ones, or I've dropped or banged them around.
Your dirt bike carb is now clean and rebuilt. Once you've done one, you'll wonder why you didn't try it sooner.
It's up to you, but I like to take a little time to clean the air filter and housing before putting the carburetor back on the bike -- I hate the thought of dust and grime getting into that shiny carb.