Try also dropping the exhaust pipe and crank her over with throttle wide open without the pipe and see if you get a pop. My '78 Batavus, the entire exhaust was very carborized. It solidifies like rock inside the exhaust. Possibly lame input but, I didn't know mine was that bad until I took it off and examined it. Also, habe you given your carb a good cleaning? Lots of tiny ports that get clogged with old gas. Definitely pull and clean your jet. Also, inspect spark plug. Replace if fouled and make sure you set your gap correctly. These little engines need a very specific gap. Good luck!