Adjusting the valves on the Rebel is super easy. I would do that in either case. But also check or replace the spark plug, and if that's not working start going up the electrical system to the coil, etc. I would expect that if it is only firing on one cylinder it means it's not a carb problem, since the carb feeds both cylinders equally. However, again the carb on the Rebel is simple, so it doesn't hurt to take it out and clean it. Be sure to check the float height (YouTube it and read the service manual), and make sure the idle mix screw is set correctly. I have been bitten by that thing multiple times on multiple bikes.