A filthy motorcycle carburettor can cause many different problems. If it's extremely dirty, the motorcycle may not start at all. If it's just mildly dirty, your motorcycle may be louder than it needs to be.
First, you need to remove the carburettor. On some models, this may be a relatively easy thing to do. However, it may be necessary for you to remove the seat, side covers, and fuel tank in order to get the carburettor off.
Use a brush to remove loose dirt and grime. An old toothbrush that you were about to throw away works just fine for this.
Remove the screws that hold the carburettor bowl on. Now you can get to the inside of the carburettor.
Using carb cleaner, which you can buy at auto parts stores, clean the inside of the bowl. If the bowl is full of grease or other foreign matter, you may need to use a stick to dig it out before you can clean the inside.
Use the tooth brush and carb cleaner to clean the other parts of the carburettor. When the springs, pins, and screws are all cleaned, your carburettor will run just like it did when it was new. Some people actually boil all of these parts in water to get rid of every last piece of grime. This is not necessary, but it will ensure that everything is clean.
Put the carburettor back together and reinstall it on your motorcycle.