The engine must be lowered from the frame. Notice that the engine is bolted to the sheet metal swingarm.. when the rear wheel goes up and down, the motor pivots with it.
The engine is free to pivot because it is attached to the frame by a single large bolt and nut. Remove that bolt, retract the center stand, and the engine / swingarm can drop to the floor. Once on the floor, the bike will balance precariously on the muffler.. Now the carb can be removed by unscrewing the 4 screws that mount the carb and intake manifold (as a unit) to the engine block. Then the carb and manifold can be seperated.
A few cables, wires and hoses prevent the engine from just dropping down. These must be disconnected to prevent stretching or breaking something. They include the decompression cable, the fuel line, the throttle cable, the rear brake cable, as well as a few electrical wires. The spark plug wire might be another one, depending on how it's routed. Some of these are attached to the engine (like the throttle cable) and some are just routed through or around the engine or frame in such a way that they interfere with the operation. Some are clipped to the swing arm.
To drop the engine, first disconnect everything that appears it might interfere . Remove the nut on the pivot bolt. Have someone hold the bike steady and, while holding the engine up with one hand, retracted the center stand with the other. Now pull the pivot bolt out. The engine is now free to fall. Lower the engine just an inch or two and rest it on blocks of wood or something (prepared ahead of time). Now look closely and see if anything else needs to be disconnected..
Once everything is ready, slowly lower the engine to the floor.
This description may sound like a lot of work and it is the first time you do it. However, when reinstalling the engine you can push it up into position _first_ , lower the center stand to support it and stick that pivot bolt in.. and _then_ re-route your cables and wires and hoses in such a way that they (most of them) will not interfere with dropping the engine the next time.. With the way my bike is now set up the whole operation takes only 3 or 4 minutes.
Accessing the carb several times is usually necessary when fixing fuel problems or during the initial hospitalization of a junkyard-jewel.. Making it easy by rerouting all that stuff is worth the effort.