I obtained an extra carb in a trade and decided to document the cleaning/disassembly/rebuild in this thread. The oem LT500R carburetor is a Mikuni TM38 flatslide design. Parts are available at Sudco along with a schematic here: http://www.sudco.com/Diagrams123/exptm.gif Just about everything is available through Sudco for around half the price that Suzuki charges with the exception of the needle. The oem needle is a 6DK3 and is only available through Suzuki as far as I know.
The tools you will need include a #2 philips and small flathead screwdrivers, 12mm wrench, 6mm wrench or socket, center punch (maybe), can of carb cleaner with the straw for the nozzle, toothbrush and rubber gloves. Latex gloves don't work well as some of the carb cleaners eat right through them. Before you begin disassembly clean the outside of the carburetor thoroughly by spraying it with carb cleaner, brushing it and then spraying it again. You don't want to have any dirt that could contaminate the inside. Goggles are a great idea when using the spray cleaner.
Start by removing the bowl and cleaning it out. Put the straw on the carb cleaner and spray down the hole in top of the bowl as shown in the picture below. You should see cleaner come out the small hole inside the bottom of the bowl. If not you will have to clean it out. This is where the choke pulls it's fuel from so if your choke isn't working properly this is probably why. It's a very small hole inside, the only thing I've found that will fit inside it is the wire from a bread wrapper.
Turn the carb over and begin disassembling the jets and float. Start with the main jet and the plastic disc under it, that will give you access to the pilot jet.
I left the slide and needle in at this point so that you could see the tip of the needle inside the emulsion tube under the main jet. If you haven't already pulled the slide do it now. Mikuni refers to the emulsion tube as the needle jet and the needle as the jet needle, for clarity I will just refer to them as the emulsion tube and needle. You can remove the emulsion tube by pushing it up toward the top of the carburetor, it should come right out. Now we remove the float hinge pin. It only comes out one way and if it's tight you may need to tap if with a center punch to get it started. Do not do this without supporting the leg on a 2x4 or something, you don't want to crack the leg. With the float arm and floats out of the way we will next pull the needle valve and seat.
Pull the screw that holds the small U-clip retainer for the needle seat and remove it. Next take off the square box that surrounds the needle and jets. Be careful removing the small wire retaining clip for the needle valve, it can go flying. Turn the carb over and the needle valve should drop out. Pull the seat straight out with pliers and the disassembly is completed.
Remove the air screw and idle screw, I like to remove the choke also but it's not necessary, you can just open it for cleaning if you want to. This is about as far as we need to disassemble the carb. You can soak the small parts such as the needle/seat, emulsion tube, jets, etc in carb cleaner while you clean the rest of the carb, if you are going to soak the needle seat remove the o-ring first. Torch tip cleaners work well for any stubborn deposits in the jets.
Refer to the pictures below when cleaning out all the orifices in the carb. For reference we will call the engine side of the carb the front, the air filter side the back, the air screw side the left and idle adjust screw side the right. You want to blow everything out with the carb cleaner with the straw on it. In some cases you may need to plug some of the holes with your thumb or finger to get cleaner through the other holes. The air hole in the rear of the carb for the pilot is a good example. With the air screw out most of the cleaner will want to come out there, if you stick your finger over that it will want to come out the bottom at the pilot jet hole. Plug that with your thumb and you will force cleaner out the small hole in the bore toward the front of the carb. Those two small holes in the bore for the pilot circuit are the two that get plugged the most often, be sure you are getting good flow out of both of them. Be sure to clean everything, the choke circuit including the feed tube sticking out the bottom of the carb, the vent holes, overflow, etc. It's important to clean everything because with things like the vents a little dirt won't affect their operation but that dirt can fall back down into the bowl, get sucked up into one of the circuits and you're back where you started.
Reassemble the carb by reversing the steps, stop before you install the main jet to adjust the float height. If you replaced the needle valve/seat it will help to put a light coat of silicone grease on the o-ring. Notice on the emulsion tube that there is a keyway cut into it so that it only goes on one way. To adjust the floats turn the carb upside down and lift the floats up so that they are not touching the float arms, I just used a couple pieces of cardboard as shims to hold them up as shown below.
The proper adjustment should be 1/2" from the float arm to the top of the spot the main jet screws into. I just cut a thin piece of cardboard and put a mark at 1/2" on it to use as a gauge as seen below. If the float needs adjusted you simply bend the small tab that contacts the needle valve whichever way it needs to go.
Install the main jet and the bowl then the only thing left to do is adjust the slide height. To adjust the slide height back the idle adjust screw out all the way so that the slide is bottomed out. Find a drill bit somewhere around 0.075" thick to use as a feeler gauge. It doesn't need to be exact this is just to get it close, anywhere from 0.070"-0.080" is more than close enough. Turn the idle screw in to raise the slide just enough that the drill bit will fit under the slide when inserted from the front of the carb.
With the slide adjusted the only thing to do is hook up the throttle cable to your throttle and make sure the cable is adjusted properly. Once you have it installed check the slide height with the drill bit again and make sure the cable didn't raise the slide. If it did you need to turn the adjuster(s) in. If that's fine open the throttle all the way with one hand and see if you can lift the slide any higher with your other hand. If the slide is not completely opening you need to turn the adjuster(s) out until it opens completely and then recheck the slide height. I believe the oem cable only has one adjustment at the throttle but some aftermarket cables have an additional adjustment in the cable itself.
That's it, make sure the air screw is set to 1 1/2 turns out and you're ready to install the carb and start jetting but that's a subject for a different thread, lol. If the carb is working properly you shouldn't have to move the idle screw much, just a little one way or the other. If you realize you need to replace the needle valve assembly or emulsion tube during inspection, or just want new parts here are the Sudco part numbers and sizes:
emulsion tube (needle jet): 003-586, R2 size
Needle valve assembly: 002-707, 3.5 size
The lt250 carburetor is very similar to this carb with a few subtle changes, I think the air and idle screws are reversed, the float is a little different design, etc. However you should be able to clean/rebuild the 250 carb following this thread also.