First suspect is always the fuel and carbs.

• The Number 1 suspect is always the fuel, ESPECIALLY if you did not buy it. Ethanol fuels can go "bad" in as little as 8 weeks. What's really happening is that the alcohol is coming out of solution with the gasoline. When the carbs gets a slug of alcohol, the engine can't possibly burn it, so you get mis-fires and sputtering. Pour the remaining fuel into your car because it genuinely doesn't care, and then go get some fresh, name-brand, high test fuel. Alcohol will also spoil the plugs, and they will NOT recover. 
• Usually bikes that look that nice have had a bath somewhere in the recent past. That's not a bad thing, except that people prefer pressure washers these days, and all it takes to upset the fuel system is 4 or 5 drops of water. Water sinks to the bottom of the float bowl and covers up the main jet, and so mid-range and above starts sputtering. Drop the float bowls on both sides one at a time. Pour the contents of the bowl through a shop towel or rag that's been folded to triple thickness. Gasoline will soak into the rag, while water will "bead up" on the rag and be plainly visible. 
• There are also 100 other small tuning points that a new owner needs to go over. It's all part of new-to-you, classic bike ownership. 98% of these "should be checked" items are in the article I pointed you to in your first post. Please go back and review all the tuning items in that article... starting with care of the auto ignition advance unit and strobe timing of the ignition timing.

Hope this helps.