MOPED ARMY

tim is right. check the gas cap vent. when the engine begins to stall, loosen the cap. if the engine recovers quickly, itsdefinetly the cap vent.

now, heres my assumption of your problem from a different view. you say it will run with the choke closed. and that it sputters. this sounds to me like an over rich condition. when you changed your spark plug, what did the old one look like?

if the gas cap vent is plugged, the engine may still run on choke because it cuts off the air flow and allows the engine to run somewhat on very little fuel and air. your plug should look whiteish or grey. maybe even very light tan.

but another problem may be a loose chunk of carbon in the exhaust. if its floating around it the pipe it may plug up the baffle tube at times. in this case, your plug would look very black and sooty.

in all honesty tho, i think your gas cap vent is plugged.

I'll recheck the gas cap, however I don't think this is the issue. I have thought of this one, and there is a stock " vent hole " which does not appear t/b blocked. When I changed the plug, it was a lovely tan color. I think the mixture is correct, but I did make an oil change recently, from Redline to Elf 100% synthetic racing oil.