Riva 180/200 muffler/rear wheel removal This process is not difficult. Remove the LH side panel. There are two 12mm bolts and two 6mm Allen bolts holding the muffler in place. First remove the two 6mm Allen bolts at the exhaust manifold. Then remove the two 12mm bolts on the muffler. You want to remove the two Allen bolts first because you don't want the muffler hanging from the tip of the exhaust pipe. It will put a lot of stress on the tip of the exhaust pipe and the exhaust port. At this point the muffler is loose. You may have to wiggle it a bit to break it free if it has been on the scooter a while. There will be a copper O-ring inside the exhaust port. This O-ring should be replaced with a new one so that you'll have a good seal when you re-install the pipe. If you really want, you can usually re-use this O-ring if you anneal it first. Heat it up with a propane torch until it's red hot, then drop it in a pan of water. This process will soften it so that it can be re-used.
To remove the rear wheel, first pop off the wheel cap with a flat blade screwdriver and remove the cotter pin on the axle. Then remove the 22mm nut on the axle. An easy way to do this is with an impact driver. If you don't have one, just use a 22mm socket/ratchet or a 22mm box end wrench. Depress the rear brake pedal with your right foot and give the ratchet or wrench a wap with a rubber mallet or small piece of 2 X 4 to break the nut free. It may be difficult to remove the nut unless you use this type of impact action, especially if the nut is on there really tight.
By the way - this info is courtesy of Jack's Scooters Yamaha page which can be found at:
http://www.jacksscootershop.com/yamaha_info.HTML