Hello,
Im not sure, but did you turn around the carb for easier throttle linkage? If i remember correctly, there is a small hole between the crankcase to allow crankcase pressure to pump fuel to the carb. If the carb is turned and the hole is blocked, the engine will run for a little after being primed, but will then die b/c of lack of fuel pressure.

It may stay running on straight gas b/c it takes more fuel to run the engine when using gas/glow than straight gas. It could also be siphining/gravity feeding gas from external tank which keeps it running.
Hope to help,
TFF