Oh yeah get the filter on there.. That might cure it. They might have warmed the bike up before you arrived to cover up a few small faults.. The carb should have a strong return spring inside that cover.. Perhaps the throttle cable is old and wasn't returning properly.. You should check the kill switch works !!

Did you ride it when you went to look at it ? If it was going fine, and you went for a ride at the point of sale, then surely it's something simple, and the carb doesn't need stripping.. If you bought it untested then that new carby is probably the problem, but it's not likely to be as simple as needing a clean.. it probably needs retuning for the bike. I'd contact the old owner and ask if they still had the original carb and could you get it please. It's easier to clean an original carb and get it running right again that retune a new after market carb that might need new jets. About the only simple adjustments are the idle mixture and speed, and the slide needle height. Your symptoms sound like it could be the slide needle, and the easy test is to take the top cover off and move the slide needle's circlip down one notch and see if that improves things. Before pulling anything off the bike though you should check the fuel tap and filter's aren't blocked by undoing the drain screw on the bottom of the carby and making sure fuel runs out freely. That checks the tap, filters and that fuel is getting into the carb fast enough. If that's good, try adjusting the slide needle. Oh.. and that side cover will be availiabe from the dealer. They have a rubber seal and will be much better than a home made one of plastic.

You can make the cover for no cost from a piece of plastic and screw it to protect the throttle from dirt!

I believe you have a carburetor issue, my guess is the primary carburetor jet is clogged. I would remove the carburetor bowl and remove the larger jet and clean the tiny hole that runs through the center of this jet as well as the tiny holes on the jets sides. You can buy the professional jet cleansing brushes or to save money find a piece of stranded wire and remove one strand and use that to clean the jet. This clogged jet can be difficult to clean and sometimes you need to buy a new jet, the size of the jet is marked on it (you'll need a magnifying glass to read it).

You should consider a carburetor clean and some fresh new gaz

Spark plug cable got knocked half off.. If you're saying it's got this problem on the trip home on the truck then it must be something simple surely. Are you sure you got the tap in the right position ? Plenty of gas? No rags stuffed up the exhaust ? (hey I don't know.. but humans do stuff like that ). Hasn't got a tap on the fuel tank breather ? Check anywhere wires might have got pinched by tie-downs.