You will definitely want to open up those carbs. After 15 years, they are bound to need internal attention. Now if the bike was stored with the float bowls drained and fogging oil shot into them, you might get by. But I wouldn't even proceed until I popped the carbs off. It really isn't as intimating as most people fear. I have not worked on a 78, but the 79's have that coil bracket/mount in the way. Just loosen the bolts and it will pivot up out of the way. Then remove the fuel line, and drain lines. Loosen the clamps on both ends of the carbs. Then remove the bolts that hold the insulators to the head. You can now pivot the insulators inward and get enough room to remove them. It may take a flat screwdriver to pry them off, there is a groove in the rubbers where the carb end fits. Then pry off the air box rubbers from the carbs.
Now there should be enough room to move the carbs to the left side of the engine and loosen the choke and throttle cables. If the throttle won't rotate, try hitting the shafts with some PB Blaster. Outside and inside. You should be able to still remove the cables, even if the shafts are frozen, it will be a bit harder tho. The carbs can almost be completely removed from the bike with the cables attached. You may have to open up some cable holders to get the slack, but they will come most of the way out. Left hand side of the engine, now.
This whole process may take close to a hour the first time, but after doing it several times (and you probably will), it only should take 10-15 minutes.
Now you are ready to disassemble the carbs which is a completely different post. I would recommend that you purchase LRCXed book on carb cleaning. It is written with the novice in mind, but also has good information for the experienced. I think you can click on his name in this forum and it has information on how to purchase.
If you get stuck anywhere, just ask for help. We can walk you thru most any problem on these. I didn't notice if you have listed your location and bike model on your profile, please do that. I may live up the street from you and don't know it.