If the engine is horribly worn out than it could behave like you describe, but if it has decent compression than it's not getting enough fuel (needing the choke to run is your clue). Your on the right track with the fuel filter, petcock, and carb-cleaning but you must have missed something. Is the float level correct? could there be a varnish buildup in the jet passages? I'm a fan of oil injection and think they should be kept in tact if possible, but I think if air gets into the system then they'll stop working and will need to be bled. best luck.