OK, Bob I'm late into your problem but ... I bought a "side of the road" MS250. Got it home and tried to start it. Nothing but a sore shoulder. Removed the housing and air cleaner. Shot a tiny bit of ether into the cylinder. Had to hold throttle wide open to keep the butterfly open. No pop. Tried a little fuel, no joy. Plug was dry. Pulled the carb. Shot ether directly into cylinder. Fired up. Took carb apart and cleaned, particularly the needle, seat and jets. Figured the guy left bad fuel, ethanol, and gummed it up. Drained his fuel, replaced it with fresh, ethanol free. Shot some into the cylinder, carb is soaking. Fired up with no carb in place. Reassembled the carb and set all adjustment screws to the middle. Discovered he had adjusted the low speed mixture screwing the screw all the way in. Shot some fuel through the carb into the cylinder, holding the throttle open. Fired up and kept running. Adjusted the carb, all is good. Thought for you. I may have missed it but I did not read that you reworked the carb or held throttle open when squirting ether or fuel. If the throttle is not open the butterfly will block any fuel getting in. That you just set it down and it quit with no wierd noise tells me it is probably something simple and cheap. I'd bet some crap got through or around the air filter or it is deteriorated, and clogged the carb. If you messed with the adjustment screws that may have made it worse. Recommend, remove carb. Shoot good fuel or ether (but it has zero lubrication so use sparingly if at all) directly in the intake. If it fires at all rebuild and adjust the carb. Hope this helps. All that complicated stuff may be overthinking when the basics, fuel and fire, rule. Good luck.