There are many reasons that an engine can run too rich, and the place to start is to be sure that running rich really is your problem. Running rich means that the fuel:air ration in your cylinders is too high. That means that there is too much gas and not enough air getting to the cylinder. The easiest way to verify that your bike is running rich is to run the bike for a little while, then pull out the spark plugs and take a look. If they are covered with dark black carbon build up, then your bike is in fact running rich. Be sure to check all four plugs to see if all your cylinders are rich or just a few of them.
If you are running rich, then you almost certainly have something improperly adjusted in your carburetors. If you are not the kind of person who likes to work with small parts, and keep your workbench organized, then working with carbs is probably not for you. But if you do like to tear things apart and get to the heart of them, then press on and we'll have your bike running right in no time.
The most likely cause of running rich is that your bowls are flooded. the bowl is the bottom part of the carb where the gas sits, and your bike has four of them. each one has a flat-head screw on the bottom side, and a nearby nipple where you can connect a piece of clear hose. Connect a clear plastic hose to the nipple, and then use a flat screw driver to loosen (but not remove) that screw. When you loosen the screw, gas will flow from the bowl into the hose. Hold the hose up to the side, and the gas level should be about even with the seam where your bowl connects to the body of the carb. If the gas level is any higher than that, then your bowls are flooded. That means that your float needle seats are not sealing correctly inside the carbs. This can be caused by having pieces of debris or rust clogging the float needle seat, or by having old worn out float needles. you can remove the carbs from the bike and check each float needle individually. The rubber tips should be cone-shaped, and come to a nice point. The brass seats should be clean and free of debris. Clean everything out with carburetor cleaner from your local napa or auto zone, and put it back together. Be sure not to get carb cleaner on your float needles or other rubber parts though as it will dissolve the float needles. If your float needles are worn, then they will need to be replaced. If the inside of the carbs are just dirty or gummy, then clean them out until they are nice and clean.
If your bowls were not flooded, then you may have a problem with the fuel jets. A lot of people take carburetors apart to clean them, and get the little brass jets in the wrong place when they put it back together. Each jet has a tiny number inscribed on its top surface. The bottom side of the carburetor (inside the bowl) has two jets that look similar and have identical threads, but the holes that go through them are not identical. The main jet is the one that goes right in the middle of the carb and screws into the brass needle jet. It's size is around 120. The pilot jet goes off to the side and its size is around 35. The threads on these jets are identical, but if you get them in the wrong place, then your bike will always run rich -- especially at idle. Check all four carbs to be sure that the jets are correct. Another good indication that you have the jets switched is if you can easily start the bike without choke even if it is dead cold.
If both the jets and the fuels levels look good, then you may have more serious problems. But those two are the most likely cause. Those are things that are easily overlooked by first-time mechanics, and they are pretty easy to fix if you have patience and are willing to deal with the small parts that go inside of carburetors.
Good Luck,
-Josh
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