Then I would move to the fuel system. Pull the petcock, and remove the screen up in the tank. They get clogged real easily. Then add an In=line fuel filter, and pull the carb. You need a can of spray carb / brake clean. Pull it apart, and clear all the passages.
The jets / ports may be clogged.
The next thing is a compression test. Maybe a burnt or sticking valve. Maybe the valves just need an adjustment.
This should keep you busy for a minute.
Since it does run, it HAS to be somewhere in this list.
You're welcome for the welcome. Thanks for saying thanks!